appDisplay: Use custom function to retrieve item page and position

appDisplay: Use custom function to retrieve item page and position

It is important that '_loadApps()' return a sorted list -- adding the
same icons at the same positions but in different orders results in
a wrong icon grid.

Add support for using a custom positioning function, and implement it
in AppDisplay. Because FolderView doesn't implement a custom sorting
function, the items are still sorted alphabetically.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1284