Add function to get all selected images

This should be used all over the code I guess instead of fiddling with
the db in many places.
Center filmstrip on first selected image in map mode

This fixes #9090
Revert "Add function to get all selected images"

This reverts commit 3ad2103f4dae9e935a0bc197d42f5fd595a4a757.
We should probably use the already existing dt_collection_get_selected
Add limit option to dt_collection_get_selected
Use dt_collection_get_selected with limit = 1
Update screenshots in apddata file
Announce changes to appdata format on devel list