appDisplay: Stop watching FolderIcon parent view when destroyed
When a FolderIcon is opened, it asks the parent view to allocate
space for it, which takes time. Eventually, the space-ready
signal is emitted on the view and the icon can make use of the new
space with its popup. If the icon gets destroyed in the
interim, though, space-ready signal handler still fires.
This commit disconnects the signal handler so it doesn't get called
on a destroyed icon.