Bug 914 - Don't allow 'alwaysontop' in fullscreen mode, always allow switching to other applications via ALT-TAB

As described @ <http://forum.jogamp.org/Regression-of-alwaysOnTopBug-tp4030735p4030738.html>,
we shall not steal the desktop in fullscreen mode via 'alwaysontop'.

Latest tests on X11/GNU/Linux and Windows7 - before this patch:

With default settings, i.e. alwaysontop (atop) disabled,
it works as expected here, i.e.:
  - ALT-TAB triggers WM dialog, switching between apps.
  - ALT-TAB can actually switch to other apps.

However, with enabled atop:
  - ALT-TAB triggers WM dialog, switching between apps.
  - ALT-TAB does _not_ switch to other apps. (*)

I consider this a serious issue, since we shall not steal
the desktop in fullscreen mode.

This patch disables atop in fullscreen mode,
i.e. (*) will switch to other apps again!