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

Cargo.lock

@@ -5644,6 +5644,7 @@ dependencies = [
  "log",
  "lru",
  "ouroboros",
+ "parking_lot",
  "regex",
  "resvg",
  "roxmltree 0.20.0",