Added checkstyle plugin to maven build (for all modules)

Right now, the only check is that interfaces have javadoc. We'll add more
checks once the API doc situation is under control.
Fixed toolbar add/remove triggers so items don't get wiped out too early (refs UF-2)

This is not the best solution for a couple reasons:

1. it's not symmetric API (place gained focus vs. place closed)
2. it doesn't track actual focus or visibility, so the toolbar will get overfilled
   when the perspective has a lot of places in it