Clean up Highlights plugin and fix test suites

Add a clean, isolated set of tests for the new Highlights plugin. The
existing tests were horrific, and extensively tested implementation
rather than interface. The new tests are much clearer.

This commit also makes the drawAll, draw, undraw, redraw methods on the
Highlights plugin public.
Clean up Adder plugin and fix test suites

Add a (relatively) clean, isolated set of tests for the new Adder
plugin.

This commit also makes show, hide, isShown public API on the Adder
plugin.