Enable Multiple Transcription Layers

_From @cubap on October 16, 2015 3:14_

Include creation and editing support for the `layer` property in`sc:Manifest` which may separate transcription annotations into trans-canvas layers of a shared purpose and potentially continuous content.
### Use Cases
1. A music manuscript may have the notes annotated in MEI terms, while the lyrics are transcribed in one layer and translated in another. This application would not offer a preference to one, as another arrangement may, but still allows for alignment.
2. A heavily glossed or commented manuscript may benefit from separate layers for the main text and the running commentary.
3. A palimpsest or similarly reused support may contain parallel annotations, which may in turn preference their own reading angles or image choice.

_Copied from original issue: cubap/TPEN3#1_