iconGrid: Merge PaginatedIconGrid and IconGrid

iconGrid: Merge PaginatedIconGrid and IconGrid

Now that the only user of the IconGrid is AppDisplay, and
it only uses the paginated icon grid, there's no point in
having the two classes split anymore.

In addition to that, future commits will introduce a layout
manager that will extend current icon grid features, and
merging PaginatedIconGrid and IconGrid in the same class will
vastly simplify this transition.

Merge PaginatedIconGrid into IconGrid, and adapt AppDisplay
to this change.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1267