Add detailed "index status" stats

Good job, thanks!

However, even though this is exactly as I described it in the IS, I have some suggested improvements:

1. Instead of making this a collapsible extra form element, maybe just display this information next to the datasources in the info table? (Perhaps in <small> tags so it doesn't draw too much attention.)
2. To avoid "breaking" an interface this late in Beta, and to not make it compulsory for tracker plugins to provide this information (it's not really necessary, just nice to have, and this might make it harder to implement some tracker algorithms), maybe the two new methods should live on a new interface, something like TrackerPerDatasourceInterface. Then just check whether the tracker implements that before using it (and otherwise just don't provide this information).

(Also, the first sentence in a doc comment should be only one line (max. 80 characters), so maybe just lose the "for this index" part for the two new methods' comments?)

Otherwise, though, this really looks very good already, so thanks again!

Sorry, should have reviewed more carefully: apparently (maybe at some point since I created this issue) getIndexedItemsCount() et al. already have $datasource_id arguments, there doesn't seem to be any need for additional methods, an additional interface and all that jazz.
Just a few lines in the theme function.

See the attached patch, and please test! (I don't have any web server running currently, so this is completely untested.)

And sorry for the confusion and unnecessary work!

Good to hear, thanks for reviewing!
Committed.
Thanks again for your work on this, alan-ps!