Remove keep-above forcing in fullscreen mode

Window managers are a pain.

Forcing keep-above for the main window in fullscreen mode breaks
fullscreening entirely in Awesome WM. Therefore, rip out this hack:
workarounds that completely break functionality that would otherwise work
will be dropped.

Xfwm4 will be annoying now if there are subwindows visible in fullscreen
mode, so it's best to use the docks on this wm and unfullscreen when using
the preferences window. I use xfwm4 myself. Sorry for the inconvenience.

Unity doesn't seem too badly affected either way these days.

Fixes #89.