Revert PR#3231 and make "visualisation as screensaver" behave as having fullscreen manually toggled (so it won't trigger screensaver state at all)

So since https://github.com/xbmc/xbmc/pull/3231 seemed to cause more irritation and troubles than it helped, this does the following change:

- Restore the old behaviour by reverting the commit
- Move the check for IsAudio+usevisasscreensaver above the screensaver-enabling processing and don't tamper with screensaving triggers when the condition applies

With this, when idle-time exceeds the screensaver wait time, one plays music and has the flag set in the options, XBMC behaves as if the user manually pushed the fullscreen key (TAB, SHIFT+Fx on nyxboard etc.) and doesn't do any screensaving handling at all. This still fixes the missing "onScreenSaverDeactivated" announcement (not required as it won't send the ScreenSaverActivated announcement anymore for musicvis-as-saver) and makes XBMC-Music users happy again as it seems they liked the old behaviour.

Probably for @jmarshallnz to review. Build-tested on Linux.