The problem is that updates of the selections can be executed only
asynchronously via KisImage's scheduler. It means that we cannot
update a selection while it is not attached to any selection mask
that is a part of image's hierarchy.

Therefore now, if we get a selection update request while the
selection is not part of the layer hierarchy, we just save this
request and mark it as "stalled". As soon as a selection gets
attached to the image, the updtate is issued.