keyboard shortcut "R" to toogle the razor tool (#4007)

* keyboard shortcut "R" to toogle the razor tool 
* toogle the razor tool: update docs
* Add setting for actionRazorTool shortcut

Co-authored-by: FeRD (Frank Dana) <[email protected]>

Author: Spyna

doc/main_window.rst

@@ -130,6 +130,7 @@ Ctrl+K              Slice All: Keep Both Sides
 Ctrl+L              Slice All: Keep Left Side
 Ctrl+J              Slice All: Keep Right Side
 Ctrl+G              Toggle Snapping
+R                   Toggle Razor tool
 Ctrl+X              Split Clip
 Ctrl+Shift+D        Thumbnail View
 Ctrl+T              Title Editor

src/settings/_default.settings

@@ -518,6 +518,14 @@
     "value": "Ctrl+M",
     "type": "text"
   },
+  {
+    "category": "Keyboard",
+    "title": "Toggle Razor",
+    "restart": true,
+    "setting": "actionRazorTool",
+    "value": "R",
+    "type": "text"
+  },
   {
     "category": "Keyboard",
     "title": "Previous Marker",

src/windows/ui/main-window.ui

@@ -884,6 +884,9 @@
    <property name="toolTip">
     <string>Razor Tool</string>
    </property>
+   <property name="shortcut">
+    <string>R</string>
+   </property>
   </action>
   <action name="actionSnappingTool">
    <property name="checkable">