Cesium registers mouse events it's not using

`ScreenSpaceEventHandler` registers a bunch of mouse event handlers when it is initialized regardless of whether Cesium is actually using these event handlers. This can cause weird side effects when Cesium is embedded in a page because it captures all mouse events that happen over the canvas.

There is also no easy way to remove a specific event handler.

Start by locating the ScreenSpaceEventHandler implementation and reading how it initializes mouse event handlers. Trace how handlers capture canvas events and determine the existing event-registration API; done means unused handlers are not registered by default and a specific event handler can be removed without affecting others.

Written by the indexing model from the issue text.