boxes: Add support for accessing markdown help popup from compose.

This commit adds a keypress event to show the markdown help menu while
a user is in the compose area, to help users to check for proper
markdown syntax while writing a message.

This is a fuller fix to zulip#623.

Test added.

Author: Ezio-Sarthak

tests/ui_tools/test_boxes.py

@@ -1260,6 +1260,14 @@ def focus_val(x: str) -> int:
                 expected_focus_col_name
             )
 
+    @pytest.mark.parametrize("key", keys_for_command("MARKDOWN_HELP"))
+    def test_keypress_MARKDOWN_HELP(self, mocker, write_box, key, widget_size):
+        size = widget_size(write_box)
+
+        write_box.keypress(size, key)
+
+        write_box.view.controller.show_markdown_help.assert_called_once_with()
+
     @pytest.mark.parametrize(
         "msg_type, expected_box_size",
         [

zulipterminal/ui_tools/boxes.py

@@ -677,6 +677,9 @@ def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
             self.view.controller.exit_editor_mode()
             self.main_view(False)
             self.view.middle_column.set_focus("body")
+        elif is_command_key("MARKDOWN_HELP", key):
+            self.view.controller.show_markdown_help()
+            return key
         elif is_command_key("SAVE_AS_DRAFT", key):
             if self.msg_edit_state is None:
                 if self.to_write_box: