QMacStyle: Generalize Cocoa control rendering

So far, we were restricted by the values of ThemeButtonKind, which
consist mostly of buttons. We want to generalize to other kind of
controls when the time arrives. Already, QPushButton with pull-
down menu is not possible to get from that enum, while its easy to
get from NSPopupButton.

Task-number: QTBUG-40833
Change-Id: I244c42c42ab595f4790050eb15ade70443e155b2
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>