Deprecate GoogleMapShowAllVisibleMarkers/bounds_changed hook in ListFilterWidgetGoogleMap

[silbinarywolf]

This code is pulling a bit too much effort by default (ie. Getting in the way of developers), so I'd like to deprecate:

• "bounds_changed" hook - https://github.com/SilbinaryWolf/silverstripe-mrfilter/blob/master/javascript/ListFilterWidgetGoogleMap.js#L277 -This can be worked around with: google.maps.event.clearListeners(map, 'bounds_changed');
• Disable automatic calling of "GoogleMapShowAllVisibleMarkers" when Lat and Lng is 0. (User code can do this in the 'GoogleMapDrawInit' hook/call)
• Remove hacky 'adjust-zoom' attribute code completely.

Ideally this functionality would be pulled out to a seperate JS file like 'ListFilterWidgetGoogleMap-Migrate.js' or similar so nothing breaks during this MAJOR revision.