don't block the user from overlapping clips
@jeff
Submitted by Jeff F.T. Assigned to Alessandro Decina @alessandro.d
Description
Currently, there is a mechanism in place to prevent the user from overlapping clips onto a single layer, because the auto crossfading/transitions system is not implemented.
However, this has a pretty serious side effect: it prevents you from reordering clips' order. So if I have clip A, B one after another on the same track, I cannot drag B towards A to put it on the left of A.
Unless you use some mad trickery like creating temporary blank layers to pass clips through them, but this is both inefficient and not discoverable.
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=589725