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/components/mod.rs

@@ -1,6 +1,20 @@
-use crate::any::AnyTrait;
+use taffy::TaffyTree;
+
+use crate::{
+	any::AnyTrait,
+	event::EventAlterables,
+	layout::{WidgetID, WidgetMap},
+};
 
 pub mod button;
 pub mod slider;
 
-pub trait Component: AnyTrait {}
+pub struct InitData<'a> {
+	pub alterables: &'a mut EventAlterables,
+	pub widgets: &'a WidgetMap,
+	pub tree: &'a TaffyTree<WidgetID>,
+}
+
+pub trait Component: AnyTrait {
+	fn init(&self, data: &mut InitData);
+}