overviewControls: Wait for all animation promises on startup

overviewControls: Wait for all animation promises on startup

Currently on startup we're just waiting for a single animation (the dash
one) being completed, while this is acceptable since the animation
duration is not shorter or longer than others, it's still not clean, as
technically we should just wait for all the pending animations of the
set before calling the startup animation completed.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3944>
(cherry picked from commit 954ef2f9efe8149b2495b5a412ee21f550835830)