Add drag-open animation

Add drag-open animation

This commit adds an animation for folders that makes clear that
they will open or expand soon. This is the case when the option to
open folders during drag operations is enabled and a user drags an
item on top of a folder.

The animation goes like this:
- Replace the folder's icon with the "folder-open" icon
- Go back to the folder's original icon
- Replace the folder's icon with the "folder-open" icon once more