Multi-line/relationship usability tweaks

[iross]

I'll be playing around with the new interface this afternoon. I'll edit with additional thoughts as I have them.

• I can't select a tag type -- the dropdown box isn't selectable
• Can't delete an arrow after adding it -- can only delete it by removing one of the boxes.
• Should we add the ability to delete sub-components? Right now if I make a box, then shift+make another box, then decide I don't want the second, I have to delete both.
• Does arrow direction matter? Or will we extract it later via a layer of (box_1_type, box_2_type) logic?

[davenquinn]

• Pushed a change that should fix the first issue.
• Relation adding and removal needs controls for sure
• Sub-component deletion: probably should add this ability but it's lower priority
• Arrow direction should point from referencing to referenced entity, but the reverse relationship can be easily reconstructed using SQL logic