Singleton SystemPalette

Each SystemPalette instance installs an event filter on the
application object. Avoid this by sharing a single SystemPalette
instance.

Change-Id: Ica9f8979b33e511c1238add3d689f380d0daa93c
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>