Left off previous commit
Fix setTextReference()

It wasn’t working for non-existent tiddlers. Also added support for
setting indexed properties of a data tiddler.
Extend tag manager to edit icons assigned to tags

Add a dropdown for choosing available images for a tag.

As part of this change we need to tag tiddlers that contain images
typed as wikitext (these are inline SVG icons) so that we can display
them in the dropdown.