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,

wgui/src/renderer_vk/text/mod.rs

@@ -3,11 +3,7 @@ mod shaders;
 pub mod text_atlas;
 pub mod text_renderer;
 
-use std::{
-	cell::RefCell,
-	rc::Rc,
-	sync::{LazyLock, Mutex},
-};
+use std::{cell::RefCell, rc::Rc, sync::LazyLock};
 
 use cosmic_text::{
 	Align, Attrs, Buffer, Color, FontSystem, Metrics, Style, SwashCache, Weight, Wrap,
@@ -15,6 +11,7 @@ use cosmic_text::{
 use custom_glyph::{ContentType, CustomGlyph};
 use etagere::AllocId;
 use glam::Mat4;
+use parking_lot::Mutex;
 
 use crate::drawing::{self};