control_look/decor: Add Flat style to haiku_extras package

* Works a bit better with dark themes
* Has some interesting alternative design elements
* MIT licensed

Change-Id: I6cfd1d4d05016fb014d515d94cff3b8c8d0298b2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4508
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>