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 set the page's focused navigable upon mouse-down events from the UI.
However, we neglected to ever clear that focused navigable upon events
such as subsequent page navigations. This left the page with a stale
reference to a no-longer-active navigable. The effect was that any key
events from the UI would not be sent to the new page until either the
reference was collected by GC, or another mouse-down event occurred.
In the test added here, without this fix, the text sent to the input
element would not be received, and the change event would not fire.
