Highlight context menu items at the top of the 2D/3D viewports

This makes it easier to notice that some menu items only appear when
specific nodes are selected.

This change applies to both 2D and 3D editors, including both plugin-based
menus and the hardcoded 2D layout/animation contextual menus.

editor/plugins/canvas_item_editor_plugin.h

@@ -230,6 +230,10 @@ private:
 	HScrollBar *h_scroll;
 	VScrollBar *v_scroll;
 	HBoxContainer *hb;
+	// Used for secondary menu items which are displayed depending on the currently selected node
+	// (such as MeshInstance's "Mesh" menu).
+	PanelContainer *context_menu_container;
+	HBoxContainer *hbc_context_menu;
 
 	Map<Control *, Timer *> popup_temporarily_timers;
 
@@ -535,6 +539,7 @@ private:
 	HSplitContainer *palette_split;
 	VSplitContainer *bottom_split;
 
+	void _update_context_menu_stylebox();
 	void _popup_warning_temporarily(Control *p_control, const float p_duration);
 	void _popup_warning_depop(Control *p_control);