Use system palette colors.  SimpleLauncherDelegate has its own "button"
rather than using the shared one, which really wasn't suitable for any other
purpose.  So now example apps can use Button, and ToolButton isn't necessary.

Change-Id: I632397f36b96a26c32a86301ddacb85d5c3221f0
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>