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.
Before this change, :hover wouldn't match anything outside the shadow
boundary when hovering elements inside a shadow tree. This was most
noticeable when hovering the text inside an input element and hover
styles disappearing from the hosting input element itself.