Add set/get functions to WidgetConfig.
libaudgui: Use WidgetConfig::get/set.
libaudqt: Fix several PreferencesWidget problems:
- Don't embed QWidgets in other classes since Qt wants to delete them.
- Make connecting to overloaded Qt signals slightly less ugly.
- Add missing edit callback for WidgetEntry.
- Correctly refill WidgetCombo on update().