Fix issue 4778

This fix hides the Active Line Highlight when there is a selection in the current editor.

It does not address the general delay with the Shift-Down-Arrow, a different problem that started showing up in the Dev version of Sprint 30.  It does, however, remove the really, really slow behavior associated with the having the Highlight Active Line turned on.  The delay should now be the same, regardless of the Highlight Active Line setting.

I'm not sure I should be setting the CodeMirror Option on every single call to cursorActivity, but I didn't notice any performance issues by doing so.  Let me know if you want me to set a flag or something to avoid excessive setOption calls.