Fix a typo in command window colors

The background property wasn't binding properly since a recent
refactoring. Related to #949.  Still need to enable VS theming on this
control
CommandMarginControllerTest was failing

The change to enable command mode paste broke this suite because it
added a dependency on ICommandMode being present.  It wasn't enabled in
the mock object and the test failed as a result.
GoTo Definition Tests

Updated the VsCommandTargetTest to include a test for GoTo definition
clearing the active selection