appDisplay: Fix cut-off folders in All view

We already take care of growing the view if open folders overlap
at the bottom, however folder popups may still end up being cut
off when opening above the source icon - if the popup is high enough,
its y coordinate will be negative and therefore outside the parent's
allocation. To fix, we can either make sure that folders pop up below
their source icon in that case, or adjust the parent grid's position
as necessary while a folder is open. This implements the latter.

https://bugzilla.gnome.org/show_bug.cgi?id=694371