Makes even more parts of gwt-user separately compilable.

(user/UI, user/DOM, Activity, Place, RequestFactory, Animation etc.)

I'll follow up with separate change for logging and junit.

I think we now have a significant amount of modularization to enable
projects to depend fairly small part of the SDK (at least from the
GWT compilation perspective). I'll stop for now until we see other
big opportunities to split up more.

My future goals are to make logging and junit lighter weight (i.e. no
enforced user dependency) and also perhaps split up java.lang from
gwt.core.

Change-Id: I8b5fee8a30489f51197579d2da9a85e785cfc635
Review-Link: https://gwt-review.googlesource.com/#/c/8106/
26 files changed