appDisplay: Improve icons of folders with few apps

Folders use a 2x2 grid of the first 4 app icons as icon - if a folder
contains less apps, we currently skip the corresponding grid positions.
As a result, the overall size request may be smaller than the one for
other icons, making the folder icon look out of place.
Fix this by always using a full grid as folder icon, using dummy actors
as necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=726322