overviewControls: Make sure thumbnails expandFraction is bound to visibility

overviewControls: Make sure thumbnails expandFraction is bound to visibility

We set the expand-fraction property of the workspace thumbnails on changes
to their should-show property. The should-show property defaults to true, and
the expand-fraction property defaults to 1. The should-show property gets
updated in the constructor of the ThumbnailsBox though, and it might become
false (that is the case when starting up the shell) during construction.

In this case we're not yet listening to "notify::should-show" in the
ControlsManager, and therefore forget to update the expand-fraction
accordingly. Fix that and ensure the expandFraction is always set to the value
we expect after hiding or showing the thumbnails (we call _updateThumbnailsBox()
from _update(), which we call in the constructor).

This should be the proper fix for the problem that 9bf550da8 (which caused
the workspace thumbnails to never show) originally tried to fix.

Fixes: 9bf550da88 ("overviewControls: Set expandFraction for ws thumbnails to 0 when hidden")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7668
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3323>