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/wlx_theme.xml

@@ -4,8 +4,5 @@
     <var key="bg_color_active" value="#44475a" />
     <var key="device_color" value="#666666" />
     <var key="text_color" value="#dddddd" />
-    <var key="slider_bg_color" value="#666666" />
-    <var key="slider_fg_color" value="#ffffff" />
-    <var key="slider_text_color" value="#000000" />
   </theme>
 </layout>