Several minor fixes to the Linux Keyboarding code

1. Clarify type of GlobalCachedInputContext member variable.
2. Handle a null object crash in DBus (similarly to elsewhere in
   IbusCommunicator).
3. Prevent automatic Ibus IME in IbusKeyboardAdaptor.
4. Handle locales that would crash in XkbKeyboardAdaptor.
5. Move instructions from WSKeyboardController TextBox to Label.
6. Select the current keyboard in the WSKeyboardController ListView.
7. Ensure desired font for testing keyboards in WSKeyboardController.

Change-Id: I44e27c5181ca8964555a9191f7e8403d4d9814a4