Bug 624546 – Modification of GDBusMessage in filter function

Allow modifying a GDBusMessage in a filter function and also add tests
for this. This breaks API but leaves ABI (almost) intact - at least
dconf's GSettings backend (the only big user I know of) will keep
working.

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



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