Changed angles to use tagged Angle class.
Merge remote-tracking branch 'origin/3.0-wip-sbt' into 3.0-bth

Conflicts:
	src/test/scala/edu/depauw/scales/graphics/ClippedImageTest.scala
	src/test/scala/edu/depauw/scales/graphics/FernDemo.scala
Removed most of the places where the GraphicPanel's graphic is changed.
Cleaning up access to the graphic in a GraphicPanel
Finished making graphic private to GraphicPanel
Preparing for merge back into master
Merged changes from 3.0-bth into 3.0-wip-sbt