Splash handling prevents nxclient signalling

[uli42]

The code in Splash.c also takes care of notifying nomachine's (old) nxclient by taking/releasing ownership of some special selections on the real X server. Unfortunately most of the code is skipped if not in x2go mode (where the selections are not needed) or in rootless mode. We need to separate that into a splash related part and a notification related part.

[uli42]

Well, that's not entirely true as nxagentShowSplashScreen() is only called if in non-rootless mode on startup. Hmm, this needs more investigation.