appDisplay: Add destroy handler for FolderIcon

appDisplay: Add destroy handler for FolderIcon

It is important that the FolderView of a FolderIcon always
gets destroyed before the AppFolderPopup, since the view
may or may not be in the popup, and the view should
get cleaned up exactly once in either case.

This commit adds a destroy handler on FolderIcon to ensure
things get taken down in the right order, and to make sure
the view isn't leaked if it's not yet part of the popup.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/628