GDBusProxy: Correctly handle unknown members when having an expected interface

GDBusProxy: Correctly handle unknown members when having an expected interface

Since it is valid for a D-Bus interface / service to add new methods,
signals or properties we must NEVER warn about unknown properties or
drop unknown signals or disallow unknown method invocations when we
have an expected interface.

So this means that the expected_interface machinery is only useful for
checking that the service didn't break ABI.

Also update the docs so it is clear exactly what it means to have an
expected interface.

https://bugzilla.gnome.org/show_bug.cgi?id=660886

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