We need to handle a mouse that is not absolute positioned (the default it
seems) and this does that (crudely for the moment until we have some sort
of grab management).
Additionally, click events are propagated to the Qemu instance as well.
We don't need to do this manually now that we have GtkPicture working
correctly with content-fit. We still want to maintain our own widget to
manage things like grabs and all that though.
