UI|Task Bar|Client: Added the Audio Settings dialog

The Audio Settings dialog replicates the options from the Control
Panel's Audio page.

Also added a specialized toggle widget for toggling cvars.
Fixed|Refactor|UI|Client: Widget improvements

- Use CVarToggleWidget in Video Settings
- Menu eats mouse position events (to avoid "leaking" them to widgets
  underneath)
- Adjusted margins for the dialog heading