User-created features taken from split reads shoud create features with introns

[robsyme]

Following on from #2034, it would be hugely helpful if split reads would create features with introns.

Ideally, dragging a read spanning an (implied) intron to the User-created annotations track would create a new annotation (or add CDS features to an existing annotation) that did not include the gapped bases.

This is an example of the undesired behaviour as of 5a3cde3:

[nathandunn]

I think this might be related to #1788 - but need to investigate further

[nathandunn]

closing in favor of #1788