Remove the duplicate Button::pressed property

Make it possible for Button to redefine BasicButton's "effective
pressed" state instead of overshadowing the pressed property.

Task-number: QTBUG-33124
Change-Id: I5b3570c51e1b2f8771f713e8f3f6982757d05770
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>