Add collection browse/free text/dragdrop source

[tomcrane]

This becomes the default picker for Sorty.

It has

• free text (like browser address bar)
• dragdrop target
• collection browser with thumb strips llike https://groups.google.com/forum/#!topic/mirador-tech/0lJ7rwVwoa0

This could be based on https://github.com/viewdir/iiif-explorer-component