appDisplay: Reorganize AppDisplay actor hierarchy

appDisplay: Reorganize AppDisplay actor hierarchy

Right now, the actor hierarchy is such that the scroll view
does not contain the actual grid. It looks as follows:

              StScrollView
                    ↓
               StBoxLayout
                    ↓
                ShellStack
                ↓        ↓
   PaginatedIconGrid     StWidget

This hierarchy can be slightly reorganized by changing it to be as
follows:

            ShellStack
             ↓     ↓
   StScrollView   StWidget
        ↓
   StBoxLayout
        ↓
 PaginatedIconGrid

This will simplify future work where the PaginatedIconGrid will be
an implementation of StScrollable, in which case we'll be able to
simply remove the StBoxLayout from there.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1191