GDBusObjectManagerServer: Allow setting :connection property

GDBusObjectManagerServer: Allow setting :connection property

.. and add a C setter to do this. Also make the C getter return a
reference since the property may be set from another thread. Also
change the constructor to _not_ take a GDBusConnection since this is
something you almost always want to do _after_ creating it. The
API/ABI break is fine as there has never been a GLib release with this
type.

https://bugzilla.gnome.org/show_bug.cgi?id=648959

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