magnifier: Request window-relative coordinates for focus/caret events

Absolute screen coordinates are impossible for Wayland clients to
provide, because the clients don't know where the window is positioned.
Some clients, such as the ones using GTK 3 were providing window
relative coordinates even when screen coordinates were requested,
while others, such as GTK 4 clients, were just returning an error for
caret events or also window-relative coordinates for focus events.

So for this to work on Wayland we have to request window-relative
coordinates and translate them to the current focus window.

To ensure the correct coordinates, we have to only consider events
coming from the current focus window. All other events are filtered out
now. As a side effect this also fixes the magnifier always jumping
to a terminal cursor whenever there was some output, even if the window
was not focused.

This also needs some special handling for events coming from the shell
itself, which should not be translated to the focus window either. As
another side effect this fixes another bug that was caused by these
events already including scaling and getting scaled again.

Fixes: #5509
Part-of: <!2301>
(cherry picked from commit 8b5d0277)