Publish changes from TreeAgentSet to allow nw ext to update graph context
publish SimpleChangeEvent on World3D resize
add tests for published SimpleChangeEvent after world resize
add TreeAgentSet tests for SimpleChangeEvent triggering
Merge remote-tracking branch 'origin/5.0.x' into issue-399
get rid of SimpleChangeEvent trait
introduce SimpleChangeEventCounter for TreeAgentSet tests

I had two very similar similar SimpleChangeEvent subscriber classes for
testing in TreeAgentSetTests and AbstractTestWorld. Now it's just one
class used in both places.
Merge pull request #400 from NetLogo/issue-399

Publish breed agent set changes for use by the NW extension