schemas: Use picture-uri instead of picture-filename

picture-filename was of type string, which can only be used for
UTF-8 strings, and some filenames are not, dependending on the
locale used.

Instead we switch to using a URI, and picture-uri as the key name,
as URIs are UTF-8 compliant strings.

https://bugzilla.gnome.org/show_bug.cgi?id=633983