Make keyboard navigation in tables work again

Events are accepted when the event handlers are called. Therefore,
QTreeView::keyPressEvent wasn't called anymore. The private shared
event handler now returns true if the event was handled.

(cherry picked from commit bb93ac0e66a4b13e7e37968a4c264c8ea7bd3be1)

• Regression-due-to: 36bf091e09b8bee6efde72e2a2647da9d6146e58
• GnuPG-bug-id: T6568