Improve iiif: startCanvas and thumbnail

[awagner-mainz]

• Add startCanvas and thumbnail data to manifests
• Use startCanvas in opening Mirador
• Use thumbnail in workDetails (and worklist?)

[awagner-mainz]

Mirador should make use of the startCanvas value of the first sequence unless it has been launched with a parameter indicating a different canvas to open...
(Low priority since Mirador by default opens the first canvas in the sequence and this happens to always be the titlepage due to the way we produce our iiif Manifests...)

[dvglc]

I will try to fix this with the upcoming v2.0 release.