Editor: Dim UI when a WindowDialog is shown.

Darkens the editor on WindowDialog popup.

This adds the following new Editor settings:

- interface/dim_editor_on_dialog_popup (true) # Enable/Disable editor dimming
- interface/dim_amount (0.6) # Percentage of how much the editor will be darkened (0-1)
- interface/dim_transition_time # The duration (in seconds) of the color blending effect (0-1), 0 is instant.

Please test this thoroughly, I haven't yet seen a case where it fails to work properly but I'm sure I didn't test all
windows of the editor :P

editor/editor_node.h

@@ -598,6 +598,13 @@ private:
 	void _tool_menu_insert_item(const ToolMenuItem &p_item);
 	void _rebuild_tool_menu() const;
 
+	bool _dimming;
+	float _dim_time;
+	Timer *_dim_timer;
+
+	void _start_dimming(bool p_dimming);
+	void _dim_timeout();
+
 protected:
 	void _notification(int p_what);
 	static void _bind_methods();
@@ -753,6 +760,8 @@ public:
 	void add_tool_submenu_item(const String &p_name, PopupMenu *p_submenu);
 	void remove_tool_menu_item(const String &p_name);
 
+	void dim_editor(bool p_dimming);
+
 	EditorNode();
 	~EditorNode();
 	void get_singleton(const char *arg1, bool arg2);