Instead of defining somewhat high level mouse actions, allow granular
control of mouse clicks and mouse down/up/move events. We will want to
simulate things like holding down a mouse button after double clicking
and then dragging the mouse to another position in the future, and this
enables that.
We were generating click events always using the primary mouse button
instead of the provided button, and with the buttons field set to that
provided button.
This returns the secondary target of a mouse event. For `onmouseenter`
and `onmouseover` events, this is the EventTarget the mouse exited
from. For `onmouseleave` and `onmouseout` events, this is the
EventTarget the mouse entered to.
