SpinBox: don't force active focus when tapping the buttons on touch

This improves the usability of SpinBox on touch devices. The input panel
is no longer immediately shown when tapping the buttons, but only when
tapping the editor area.

Change-Id: I09a2218419490cb33da3b0733c9a42f871edc965
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>