viewSelector: Merge WINDOWS and APPS pages

viewSelector: Merge WINDOWS and APPS pages

Add them both in a StBoxLayout subclass with a vertical layout. This
new ActivitiesContainer class already contains an adjustment controlling
the transition between workspaces and app grid states, and althought it
is internal to it, it'll be easy to integrate with gestures in the
future.

Notice that AppDisplay is added before WorkspacesDisplay. That's because
we want the paint order to paint WorkspacesDisplay on top of AppDisplay.

Switch the ViewsPage enum to call this page ACTIVITIES, and adjust the
only caller in OverviewControls to it. At last, rename '_appsPage' to
'_activitiesPage' to also reflect the name change.

The usefulness of organizing this code in pages is lost here, but this
is a transitional state, and pages will be removed in future changes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1593>