Store parent window in QPointer
Action takes an argument to a parent window. The lifespan of this is outside the control of the Kauth action.
As this operates as a QSharedData internally it's difficult for callers to keep track of actions and update appropriately.
This adds a QtGui dependency to KAuthCore, but we had QWindow in the public API anyway.
I can pivot to an alternative where we static cast to and from QObject, if that's actually problematic.
See plasma/powerdevil!297 (merged) for rationale