Refresh the available list of libraries whenever the Import Library menu is expanded

This way the Arduino IDE doesn't need to be restarted whenever a user drops a new library directory into Documents/Arduino/libraries nor the sketchbook's libraries folder.

Note: I'm not sure how slow the rebuildImportMenu call can get if there are tons of libraries, but I expect it won't be so bad, since it's just a few stat() calls. Perhaps we could guard it with a setting (preferably defaulting to true [rebuild]) if it proves too slow.