A first try at implementing the raise/hide checkbox in adv. prefs.

- add new property to AudioscrobblerSettings to store the boolean flag
  indicating whether the hotkey is enabled or disabled
- add necessary code in AdvancedSettingsWidget to tie this new
  property to the UI in the application's preferences dialog
- add new method unsetRaiseHotKey to Application to disable the hotkey
  if needed
- modify the AdvancedSettingsWidget.ui to have the checkbox enabled
  without a hard-coded checkbox value