Mouse KCM: Align spin boxes, port to Qt5

Setting the individual spin boxes' sizePolicy to MinimumExpanding
and enclosing the QFormLayout in an outer layout with stretch to
the right, so that they line up with each other but do not extend
all the way to the right. As suggested by davidedmunson.

Differential Revision: https://phabricator.kde.org/D2853