(cherry picked from commit 21ffd90556103f3f1a045a9c8216926401c62ff3)

Fix #155104 by Scott Falconer: Add "Zoom to cluster area" support.

Allow falling back to the default Clusterer marker if desired.

Allow entering larger numbers in some marker manager settings.

Allow choosing between the original and the new "zoom to marker area" behavior on Clusterer.

* Remove references to mousewheel.js, clean up variable.
* Add support for Continuous Zoom, enabled by default.
* Add behavior flag for disabling Continuous Zoom.
* Remove workaround from may 2007 (rev 1.1.2.15) that was causing zooming problems.

Third party code, even GPL, doesn't go in -contrib.