Remove geometry specific event type

In terms of "API" the major remaining difference between geom in `master` and geom in `vector-api` is in the custom geometry events.  I quote "API" because there are a ton of differences in the geometry implementation (flat coordinates in particular).  And while there are methods in `vector-api` that are not in `master` and vice-versa, the most significant difference in our use of geometries is that `master` relies on `ol.geom.GeometryEvent` instances that carry "dirty" extent values on geometry changes.

As part of the continued exercise in minimizing differences between the branches, I'm going to get rid of `ol.geom.GeometryEvent`.  This will result in some awkwardness (e.g. features storing copies of geometry extent), but I still think it is an incremental change in the right direction.

This is currently a work in progress.  I'll add a comment when it is actually ready for review.