Make moz-icon urls support both a nested file url and a non existent dummy file.
i.e.
moz-icon:file:///c/test.html?size=32
and
moz-icon:dummyFile.html?size=16
in the later case we use the file extension to determine the icon to fetch.
turn icon urls into moz-icon to avoid name space problems.
the format is now:
moz-icon://file://some file?size=16&contentType=text/html
The content type parameter is optional. The file need not exist.