FolderView Positioner: fix dragging files across screens.
f0ede2d8 fixed a bug in which items would disappear when dragging between screens. The rationale in the description was correct. The fix there was to queue the connection of rowsInserted so that we don't emit move signals before Positioner proxy model has processed any pending rowsInserted signals.
This was incorrectly simplified in 99a35972 . The important part isn't to process it after FolderModel gets a source model set, but to process it after Positioner gets the source model, which FolderView doesn't know about and the original bug introduced.
Rather than a straight revert the code is adjusted to emit the move signal delayed. This is safer than delaying the processing of a signal with index arguments.