appDisplay: Special-case dropping over page indicators

appDisplay: Special-case dropping over page indicators

This brings back the ability to drop an icon beyong the last page.
Because the acceptDrop() method does not have access to the target
drop actor, to avoid an extra pick or manually calculating it from
allocations, simply store it during DragMonitor.dragDrop(), and
use the target actor in acceptDrop().

This commit also removes the last usage of SidePages, so drop the
enum too.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2335>