preliminary support for inline commentary

@caesarfeta this is the start of support for inline commentary. Still need to add pagedown to process Markdown in the inline text but since Yannis didn't actually use markdown to create the commentary that seems less of a priority.  We might want to at least format the links though. In fact, these aren't really commentary annotations being handled this way - they are quotations but because they aren't using the CTS-enabled links (because these sources weren't available to him that way) this seemed the best way to represent them for now.

For the display I'm reusing the same window as for the External annotations (if we use this code going forward that plugin should probably be named something more general).

I struggled a little bit with the ui paradigm for activating the link. These annotations can conflict with the alignment annotations. This is actually also a potential problem with the external links so we might need to rethink that too.  I left those as-is for now, for these I'm using a footnote-type of link at the end of the span and not highlighting the words in the span. We should probably highlight the words in the span, but I'm also not sure if I have the spans right for these links (I guess based on Yannis' document). 

Let me know if you want to talk about any of this before merging it.

I also just realized I still owe you a fix for the milestones...