Fix #676108. Original patch by greenSkin, heavily modified by me to hopefully maximize compatibility.

In theory, this should work in both keys_api and keys now.

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.

Remove the markerdata hack.
Add a marker file generator and fix markers once and for all.
Add a button to the settings page to regenerate the marker cache.

Make a slight change to how gmap element processing works. This should reduce warnings.
Add some tests.