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/Cargo.toml

@@ -18,6 +18,7 @@ image = { version = "0.25.6", default-features = false, features = [
 log = { workspace = true }
 lru = "0.14.0"
 ouroboros = "0.18.5"
+parking_lot = "0.12.4"
 regex = "1.11.1"
 resvg = { version = "0.45.1", default-features = false }
 roxmltree = "0.20.0"