Summary:
The OnScreenNotification gets moved during its placement. This update
of the geometry did not get synced to the QWindow's geometry as it
should.

The reason for this is the rather special implementation of
AbstractClient::move. This just modifies the geom variable and does not
call into setGeometry at all. Instead a virtual doMove method is invoked
to inform the platform specific client implementation about the move of
the window. For a "normal" ShellClient that is fine - there is nothing
to do.

This change adds an implementation of doMove in ShellClient to sync the
geometry to the internal window. This is not yet enough to fix the
geometry of the OnScreenNotification. It gets placed with a
GeometryUpdatesBlocker in place causing the doMove never to be called.
Instead setGeometry is invoked again when the geometry updates get
unblocked. So far ShellClient did not do anything in this case. This
change ensures that at least the doSetGeometry method is performed again.

Test Plan: Added test case and manual testing of OnScreenNotification

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3963