overview: Implement startup animation

overview: Implement startup animation

The new startup animation consists of rising the Dash from the bottom,
falling the search entry from the ceiling, and going from HIDDEN to
WINDOW_PICKER with an opacity applied.

One little trick from IconGridLayout was added to ControlsManagerLayout,
which is a promises-based wait for allocation. This is required to make
sure that the transformed position of the search entry is valid, which
is only the case right after an allocation.

This animation also ensures that the overview is shown right on startup.

For session modes that do not have an overview, continue using the same
fade + scale animation.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1678>