Bug 621945 – Filter outgoing messages in GDBusConnection

This patch breaks some rarely-used public API (only known user is
dconf).

This patch is based on work from Peng Huang <shawn.p.huang@gmail.com>.

See https://bugzilla.gnome.org/show_bug.cgi?id=621945



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