Improve the UI for changing the egui theme (#4257)

I added a new demo - a `Frame` editor:

![frame-editor](https://github.com/emilk/egui/assets/1148717/d0bec169-c211-45a3-9f53-5059fb8fc224)

This whole menu is now just a a bit nicer to use:
<img width="406" alt="Screenshot 2024-03-28 at 09 49 16"
src="https://github.com/emilk/egui/assets/1148717/32d12067-7cf0-4312-aa12-42909a5ed5ac">

Author: emilk

crates/egui/src/containers/frame.rs

@@ -52,6 +52,7 @@ use epaint::*;
 /// Note that you cannot change the margins after calling `begin`.
 #[doc(alias = "border")]
 #[derive(Clone, Copy, Debug, Default, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 #[must_use = "You should call .show()"]
 pub struct Frame {
     /// Margin within the painted frame.

crates/egui/src/context.rs

@@ -1567,7 +1567,7 @@ impl Context {
 
     /// The [`Style`] used by all new windows, panels etc.
     ///
-    /// You can also change this using [`Self::style_mut]`
+    /// You can also change this using [`Self::style_mut`]
     ///
     /// You can use [`Ui::style_mut`] to change the style of a single [`Ui`].
     pub fn set_style(&self, style: impl Into<Arc<Style>>) {

crates/egui/src/introspection.rs

@@ -150,23 +150,33 @@ impl Widget for &mut epaint::TessellationOptions {
                 validate_meshes,
             } = self;
 
-            ui.checkbox(feathering, "Feathering (antialias)")
-                .on_hover_text("Apply feathering to smooth out the edges of shapes. Turn off for small performance gain.");
-            let feathering_slider = crate::Slider::new(feathering_size_in_pixels, 0.0..=10.0)
-                .smallest_positive(0.1)
-                .logarithmic(true)
-                .text("Feathering size in pixels");
-            ui.add_enabled(*feathering, feathering_slider);
+            ui.horizontal(|ui| {
+                ui.checkbox(feathering, "Feathering (antialias)")
+                    .on_hover_text("Apply feathering to smooth out the edges of shapes. Turn off for small performance gain.");
+
+                if *feathering {
+                    ui.add(crate::DragValue::new(feathering_size_in_pixels).clamp_range(0.0..=10.0).speed(0.1).suffix(" px"));
+                }
+            });
 
             ui.checkbox(prerasterized_discs, "Speed up filled circles with pre-rasterization");
 
-            ui.add(
-                crate::widgets::Slider::new(bezier_tolerance, 0.0001..=10.0)
-                    .logarithmic(true)
-                    .show_value(true)
-                    .text("Spline Tolerance"),
-            );
-            ui.collapsing("debug", |ui| {
+            ui.horizontal(|ui| {
+                ui.label("Spline tolerance");
+                let speed = 0.01 * *bezier_tolerance;
+                ui.add(
+                    crate::DragValue::new(bezier_tolerance).clamp_range(0.0001..=10.0)
+                        .speed(speed)
+                );
+            });
+
+            ui.add_enabled(epaint::HAS_RAYON, crate::Checkbox::new(parallel_tessellation, "Parallelize tessellation")
+                ).on_hover_text("Only available if epaint was compiled with the rayon feature")
+                .on_disabled_hover_text("epaint was not compiled with the rayon feature");
+
+            ui.checkbox(validate_meshes, "Validate meshes").on_hover_text("Check that incoming meshes are valid, i.e. that all indices are in range, etc.");
+
+            ui.collapsing("Debug", |ui| {
                 ui.checkbox(
                     coarse_tessellation_culling,
                     "Do coarse culling in the tessellator",
@@ -178,12 +188,6 @@ impl Widget for &mut epaint::TessellationOptions {
                 ui.checkbox(debug_paint_clip_rects, "Paint clip rectangles");
                 ui.checkbox(debug_paint_text_rects, "Paint text bounds");
             });
-
-            ui.add_enabled(epaint::HAS_RAYON, crate::Checkbox::new(parallel_tessellation, "Parallelize tessellation")
-                ).on_hover_text("Only available if epaint was compiled with the rayon feature")
-                .on_disabled_hover_text("epaint was not compiled with the rayon feature");
-
-            ui.checkbox(validate_meshes, "Validate meshes").on_hover_text("Check that incoming meshes are valid, i.e. that all indices are in range, etc.");
         })
         .response
     }

crates/egui/src/lib.rs

@@ -431,7 +431,7 @@ pub use epaint::{
     text::{FontData, FontDefinitions, FontFamily, FontId, FontTweak},
     textures::{TextureFilter, TextureOptions, TextureWrapMode, TexturesDelta},
     ClippedPrimitive, ColorImage, FontImage, ImageData, Margin, Mesh, PaintCallback,
-    PaintCallbackInfo, Rounding, Shape, Stroke, TextureHandle, TextureId,
+    PaintCallbackInfo, Rounding, Shadow, Shape, Stroke, TextureHandle, TextureId,
 };
 
 pub mod text {

crates/egui/src/memory.rs

@@ -278,13 +278,15 @@ impl Options {
             });
 
         CollapsingHeader::new("✒ Painting")
-            .default_open(true)
+            .default_open(false)
             .show(ui, |ui| {
                 tessellation_options.ui(ui);
-                ui.vertical_centered(|ui| crate::reset_button(ui, tessellation_options));
+                ui.vertical_centered(|ui| {
+                    crate::reset_button(ui, tessellation_options, "Reset paint settings");
+                });
             });
 
-        ui.vertical_centered(|ui| crate::reset_button(ui, self));
+        ui.vertical_centered(|ui| crate::reset_button(ui, self, "Reset all"));
     }
 }