Bug 924: Fix X11 Fullscreen ALT-TAB Regression ; Make ALWAYS_ON_TOP Persistent when Reparenting (child -> top)

- X11 Fullscreen ALT-TAB regression (no task-switcher visible)
  We still need the fast reconfigure path for ALWAYSONTOP only toggle.
  It was removed w/ commit c9fcc8cd510abc0cbebb872dc3e457327655e778,
  which disabled toggling FULLSCREEN and ALWAYSONTOP.

  Note: Another bug in Unity WM's Compiz:
    'Unredirect Fullscreen Windows := True' disables
    the ALT-TAB taskbar/launcher HUD rendering and subsequent task-switch.
    Hence it shall be disabled .. nothing we can do here.

- ALWAYS_ON_TOP not persistent when child -> top
  WM removes ALWAYS_ON_TOP state when reparenting,
  hence reinforce it when Child -> Top.