#382 Do not make new transcript by dragging a collapsed tier

[Brian O.]

AE55) Don't let users create new transcript by dragging a collapsed tier. This now pops up a warning message (fixed by NH, 3/2005). Mark made it so that if you do this, you don't end up with overlapping/identical exons.

Maybe we shouldn't allow dragging of collapsed tiers to create new
annotations or add exons to annotations. Maybe some visual indication of "No"--Sima suggests tier shaking back and forth.