[racket] DrRacket online check syntax

I've just pushed an extension of DrRacket to support online check
syntax that I demoed at RacketCon. (This will not be in the upcoming
release by in the release 3 months from now.) The new little blue (and
sometimes green) dot in the bottom corner of the drracket window lets
you know when it is working (as will your CPU load meter). If the dot
turns red, then it was disabled for some reason (eg, your program
tried to write to the filesystem during expansion) and you can mouse
over the dot to find out why.

In addition to adding user-level functionality to DrRacket, there are
two new API-level additions. To add other online information into the
DrRacket buffer see the docs for
drracket:module-language-tools:add-online-expansion-handler. To use
check syntax programmatically, see (the new) chapter 8 in the tools
manual.

Please let me know if you run into any problems. I have seen it get
stuck such that the blue dot never turns off, but there is no CPU load
and nothing seems to be happening. I've added some logging (at the
info level); if you see that happening, please look at your log and
send me the end of it as it may have a clue to what is going wrong.
(If, by some miracle, you can make it get stuck, please tell me that
too.)

Robby