remove instance of 'the the' in various places
when extension not found, offer to take user to extension downloads page

closes #328
minor cleanup
Merge remote-tracking branch 'origin/5.0.x'

Conflicts:
	src/main/org/nlogo/agent/AgentSet.java
remove some 3D vestiges
remove some 3D vestiges
remove some 3D vestiges
whoops, make file name match class name
Merge remote-tracking branch 'origin/master' into core

Conflicts:
	docs/3d.html
	docs/dictionary.html
	docs/faq.html
	docs/interface.html
	docs/matrix.html
	docs/programming.html
	docs/systemdynamics.html
	docs/tutorial3.html
	headless/src/main/org/nlogo/workspace/ExtensionManager.java
	src/main/org/nlogo/app/App.scala
	src/main/org/nlogo/gl/render/Renderer.java
Merge remote-tracking branch 'origin/core' into headless

Conflicts:
	headless/src/main/org/nlogo/api/DummyCompilerServices.scala
	headless/src/main/org/nlogo/api/DummyParserServices.scala
	src/main/org/nlogo/api/DummyCompilerServices.scala