Highlight using overlays.

Unfortunately, if you undo, this leaves the overlays lying around, so
this seems to be the wrong way.
Propertize text instead of using overlays, so they are removed when the user does 'undo'

This has the _other_ annoying side effect that the hide/show of
existing hightlights will mark the buffer as changed.
Only support a single "remove all highlights". Toggle highlights TBD.

Lots of rename defuns
Docs
Merge branch 'dev-highlight-overlays' into dev