Sequence feature viewing

[cjmyers]

SequenceAnnotations with Roles but not Component are sequence features. These tend to simply annotate parts of a sequence and can overlap with each other. In SBOLDesigner, it sorts these so that those that are completely subsumed by others are not shown. You can then browse into larger sequence feature annotations to see those contained within them. We should consider adding this capability to SBOLCanvas.