Issue #4413

Adds an option to keep the active line highlighted even when text
inside the line is selected.

This patch enables styling of active line numbers by
adding a `CodeMirror-activeline-gutter` class to the
line's gutter space.

Closes #3040

This fixes non-empty selections having 2 highlights: the selection highlight
and the active-line highlight. Non-empty selections should only have the
selection highlight.

Bumping them for every release just adds too much git
noise and history blowup.

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.