wgui: Refactoring, various changes (see desc)

- use parking_lot for mutex (less restrictive and easier to use)
- simplify event callbacks and widget type casting
- defer component initialization at start (required for setting the initial state of sliders)
- fix non-working scroll events
- update testbed.xml
- replace slider with the real one in bar.xml
- show slider text on its handle

wgui/src/drawing.rs

@@ -116,7 +116,7 @@ fn draw_widget(
 		return;
 	};
 
-	let mut widget_state = widget.lock().unwrap();
+	let mut widget_state = widget.lock();
 
 	let transform = widget_state.data.transform * *parent_transform;