folderIcon: Move app icon loading to FolderView

folderIcon: Move app icon loading to FolderView

Future patches will diff the old and new icons of views, in order to
animate them when necessary (e.g. adding an app icon to a folder, or
from a folder back to the app grid). In order to do that, all views
must be streamlined in how they load app icons.

Currently, FrequentView and AllView are already following the behavior
expected by BaseAppView, but FolderView isn't. Its icons are loaded by
FolderIcon, and FolderView doesn't implement BaseView._loadApps(),
which makes it impossible to diff old and new apps.

Move the app icon loading routine from FolderIcon to FolderView, by
implementing the _loadApps() method.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/645