Spotimcgui: Refresh window if onInit() gets called again.
Improved initialization/shutdown process and error handling.

Handle all exceptions originated on the initialization process, which
also prevents SystemExit exceptions from bubbling to the top and exiting
the XBMC instance.
Changes on patform detection.

Use XBMC's conditional tags to detect the platform we are running on,
instead of doing custom checks.
Fixed deprecation warnings.