Manifest start canvas

[stephenwf]

Extra compatibility for Presentation 2 to 3, need an api for manifest.getStartCanvasId and manifest.getStartCanvas that can map to presentation 3s equivelent: manifest.getStart and manifest.getStartId

In presentation 2 this is an API on the sequence. To build Presentation 2/3 compatible APIs we need to be able to treat the sequences both as a range and also as part of the manifest. So the manifest API would have to reach into the first sequence to get the start canvas. A limitation of presentation 2 and 3 compatible apps, we won't support multiple sequences with multiple start properties, unless they are accessed from ranges.

Will probably cause other compatibility issues, since the resource types may be different, so selectors may be incompatible or.. funky. Shouldn't affect the GraphQL implementation (provided @type exists on resources)