AbstractApplicationWindow: Clean up bindings, fix reparenting of drawers

Querying QQC2.Overlay.overlay attached property on a Binding object was clearly wrong. It just resulted in null at all times, so basically those Bindings never took effect.

Since when: condition does not make much sense (first, those properties could never be undefined; second, why reparenting only when visible?), let's demote them into a property change handlers with null guards.

For other bindings, pointless when check was removed as well as deprecated (and yet again pointless) restoreMode.

BUG: 473214
Co-Authored-By: Jack Hill jackhill3103@gmail.com

Replaces !1326 (closed)

I've been looking into these bindings recently, and within the scope of improving Kirigami Drawers there were very close to top on my TODO list anyway.