This reduces the code we have and implicitly fixes it under wayland.

Tracking of which windows are minimised by us is now done in JS, and
through use of model indexes instead of X window IDs, which wouldn't
really have worked under wayland.

Potentially we may have some minor behavioural differences on X with
regards to which windows are managed, as the old code did not skip those
with skip_taskmanager.

Requires a tiny patch in libtaskmanager to export persistentmodelindexes to
QML.

Under wayland:
Had 3 windows, one minimised already
Clicked button. All minimised.
Clicked again, two restored