Fix memory leak in 'Ui' classes

Auto-generated QtDesigner form is supposed to be used as a parent
for our class, not as a member. Therefore, it never belongs to QObject
hierarchy, and therefore, is never auto-deleted. If you add autogenerated
Ui class as a member, don't forget to make it QScopedPointer to make sure
it is deleted on widget destruction.

CC:kimageshop@kde.org