Bug 624546 – Modification of GDBusMessage in filter function

Rework filter functions as per

 https://bugzilla.gnome.org/show_bug.cgi?id=624546#c8



This commit breaks ABI. However, this ABI break affects only
applications using filter functions. The only known user of is dconf.

Signed-off-by: David Zeuthen <davidz@redhat.com>