Improve branch indicators in common style

Improve branch indicators in common style

The branch indicators in common style was painted based on motif
style, effectively making it useless.

Since most styles inherit from Windows style, I
moved that into common style. It was already duplicating
most of the code used in commonstyle.

Change-Id: I1a383d90254bb97aebc038fa058ec0f225cf9dab
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>