Map center update now happens on moveend instead of dragend

Using `dragend` is insufficient when the map has inertia movement on.

If the map is dragged with force, the center isn't updated to the final resting place properly. Using `moveend` resolves this. This also removes the need for the separate `zoomend` watcher as `moveend` covers all movement.

In this request I removed `zoomend` as the `moveend` seems to cover both cases well. The `zoomend` had much more in it than the old `dragend` and I want to make sure that I haven't removed anything important here. Feedback encouraged. I can make any needed changes.