Add Support for Feature Annotations

[cjmyers]

Would be useful to be able to add Feature Annotations. These are regions that provide an SO Role to a location on the sequence, but they do not reference a sub-part.

[cjmyers]

Allow "Part Type" to select "Feature". When this is selected, the SBOL serialization create a SequenceAnnotation with a role, but it does not create a Component linked to a ComponentDefinition. This also would not allow "Import Component".

[cjmyers]

Should add an option to toggle visibility of features.

[cjmyers]

CD Top
Comp: Part1
Comp: Part2
SA: (Part1 at 1-4)
SA: ("omitted detail" at 5-8)
SA:(Part2 at 9-12)
Seq: aaaaggggtttt

CD Part1
Seq: aaaa

CD Part2
Seq: tttt