Add two more signals to ZathuraRenderRequest to notify pages if they are cached
or not. This allows us to move some logic away from
cb_view_vadjutment_value_changed to more appropriate places.

ZathuraPageWidget will now release the surface if
* it gets the signal that the page is no longer cached and the page is
  invisible,
* the page is not cached and the render request is aborted.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>