iiif viewer plugin released to add to PKPs plugin gallery

[stutzmann72]

No description provided.

[asmecher]

Thanks, @stutzmann72! I've made several minor comments:

• ub-unibe-ch/iiifViewer@81e46ab#r172998597
• ub-unibe-ch/iiifViewer@cac8209#r172998696
• We generally recommend maintaining branches with the same conventions as OJS/OMP/OPS (stable-3_4_0, stable-3_5_0, etc)., and releasing plugins from those branches separately. I see you've got 3.4.0 and 3.5.0 compatibility in the same branch; that's fine, but you may find it's tricky to avoid accidentally breaking e.g. 3.4.0 support if you're working on 3.5.0 developments.
• ub-unibe-ch/iiifViewer@cac8209#r172998848

None of these are at all show-stoppers for getting this plugin into the Plugin Gallery, but let me know if you'd like to address anything before I merge this in.