Support parent window and focussing on Wayland

Allows the Polkit prompt to parent itself to the respective origin
window (using XDG Foreign) and actiate itself (using XDG Activation).

Two new DBus methods are introduced to set the window handle and
activation token respectively.

The code now also handles the tokens being provided asynchronously,
i.e. after the dialog has already been shown.

While at it, re-implement parent handling on X which seemed to have
gone missing in the QtQuick port.