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

uidev/assets/gui/dashboard.xml

@@ -11,7 +11,7 @@
     <rectangle position="absolute" color="#333333" width="100%" height="100%" />
 
     <!-- left/right separator (menu and rest) -->
-    <div flex_direction="row" width="100%" height="100%" gap="8">
+    <div flex_direction="row" gap="8" width="950" height="550">
       <!-- LEFT MENU -->
       <div id="menu" width="48" min_width="48" max_width="48" height="100%" align_items="center" justify_content="center">
         <rectangle