appDisplay: Schedule relayout after adaptToSize on app folder icon grid

AppFolderDialog was calling adaptToSize from its alloc vfunc, which changed the spacing of the icon grid after its size used to calculate the adjustment for scrolling had already been determined. This was resulting in the app folder not being able to scroll all the way to the end the first time it has been opened.

Fix this by scheduling a relayout. This however can not be done immediately after the adaptToSize call on the iconGrid, because this is called from within an alloc vfunc. So instead use Meta.later_add to ensure it gets called after the alloc, but before the next redraw.

Fixes #2535 (closed)