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