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/renderer_vk/context.rs

@@ -53,8 +53,8 @@ impl RendererPass<'_> {
 		self.rect_renderer.render(gfx, viewport, cmd_buf)?;
 
 		{
-			let mut font_system = FONT_SYSTEM.lock().unwrap();
-			let mut swash_cache = SWASH_CACHE.lock().unwrap();
+			let mut font_system = FONT_SYSTEM.lock();
+			let mut swash_cache = SWASH_CACHE.lock();
 
 			self.text_renderer.prepare(
 				&mut font_system,