Issue #778

Issue #2007

Issue #1668

Rather than polluting the top-level of the editor object

Issue #1523

Addresses issue #1484 and speeds up mark-selection add-on by
maintaining text selection coverage with small chunks of text marks.

 - When mark-selection is deactivated the mark was not removed.
 - When mark-selection is [re]activated it immediately calls
   updateSelectedText() to set the mark if there's already a selection.
 - In addition to removing the mark, the handle is completely deleted
   to leave the CodeMirror instance in a clean state, and avoid re-deleting
   the mark if it's activated again.