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

wlx-overlay-s/src/gui/panel.rs

@@ -5,8 +5,8 @@ use vulkano::{command_buffer::CommandBufferUsage, image::view::ImageView};
 use wgui::{
     event::{
         Event as WguiEvent, EventListenerCollection, InternalStateChangeEvent, ListenerHandleVec,
-        MouseButton, MouseButtonIndex, MouseDownEvent, MouseLeaveEvent, MouseMotionEvent,
-        MouseUpEvent, MouseWheelEvent,
+        MouseButtonIndex, MouseDownEvent, MouseLeaveEvent, MouseMotionEvent, MouseUpEvent,
+        MouseWheelEvent,
     },
     layout::Layout,
     parser::ParserState,