Always load Startup.xml

Currently XBMC skips Startup.xml when a custom startup window is defined in settings.
This could lead to issues since skins depend on whatever they've defined in Startup.xml

This PR fixes that making sure xbmc loads Startup.xml on startup and by introducing a System.StartupWindow infolabel that skinners should user in their Startup.xml


Fixes #13445

Not for Frodo.