KMultiTabBar: do not display checked buttons in QStyle::State_Sunken

QStyle::State_Sunken is rather intended to reflect that input device buttons which are mapped into the UI element (like mousee button or space key) is currently pressed down. Styles do their painting according to that, so things have looked a bit too colorful .

Also remove QStyle::State_Raised state on underMouse() condition, as this seemed needed with QStyle::State_Sunken, though the API docs tell this flag should not be used when QStyle::State_AutoRaise is applied, like it is here.

As result the docker/toolview tabbar now looks closer also to the one of KDevelop. In a perfect world Qt would have a proper QWidget subclass and styles would render this thing better, but this might is what we could do for now.

Before:

After:

@cullmann @dhaumann @tfry (do not have RkWard around for an example, but Kate examples should give a first idea)