The content thumbnailers for each frontend were being provided the
contents url. This was only ever used to call the urldb thumbnail
setting API.
This changes it so the single callsite that passed a valid url adds
the bitmap to that url itself in desktop_history.c instead of forcing
every frontend to require the urldb API.
Additionally the old API could pass the url as NULL which was causing
asserts where this was not an expected parameter value. Because of
this this fixes bug #2286 which was also present in the monkey
frontend as both called nsurl_access() on the url without the NULL
check and caused an assertion.
