Refactor discrete zooming into delayed continous zooming

The discrete zooming capabilities could be improved significantly. The problems with apple’s magic mouse (#4410) and smooth-scrolling touchpads are becoming greater; the scrollwheel events have a time delay right now; and other map libraries allow for keeping the zoom buttons pressed for a smooth zooming effect.

IMHO, all the zooming handlers should rely on the same method: L.Map._move, the one that Map.Handler.TouchZoom uses right now. Ideally this would allow for better zoom snapping and a scroll-zooming with faster UI response.

The idea is to make a scroll-zoom interaction the same as a touch-zoom interaction: when it starts, fire a zoomstart, fire zoom on each wheel event, and fire zoomend (and snap) after the wheelDebounceTime timeout.

Similar approach for keyboard or L.Control.Zoom: fire zoomstart on keydown/mousedown, fire zoom every frame, then zoomend (and snap) on keyup/mouseup.

Unless the Safari issue is very pressing, this can be pushed back into the queue.

See also #4610

[AdityaAnand1]

Hey there, any plans to work on this?

Also, can anybody share a good smooth scrolling configuration (or link to one)? (one like Google maps is probably what we all want)

[Falke-Design]

@AndreasAtakan thank you for sharing. This is a nice plugin.