split UIContext into SendEventService and InteractionService

Trying to avoid a big collection of unrelated methods.
Merge pull request #186 from sbt/wip/havocp-split-uicontext

split UIContext into SendEventService and InteractionService
Merge remote-tracking branch 'origin/master' into wip/havocp-remove-serialization-registry

Conflicts:
	server/src/main/scala/sbt/server/UIShim.scala