breakManager: Switch from lightbox to a Clutter effect for fading out

breakManager: Switch from lightbox to a Clutter effect for fading out

From some testing on a full session with break reminders enabled, it
seems that it’s possible to break the stacking order of the lightbox
actor if the user continues to interact with the session while it’s
visible (for example, while they’re supposed to be taking a movement
break).

Rather than try and work out what’s going on with the lightbox here, it
seems more robust to switch to using a Clutter effect, like we did with
the screen time UI (in `timeLimitsManager`). Then the effect can be
applied to the main UI group, and doesn’t depend on a separate actor
being placed in the right place in the scene graph.

So, remove the lightbox, and instead add a
`ClutterBrightnessSaturationEffect`, and use its brightness property to
fade the screen slightly while in breaks.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3655>