GDBus has always supported matching strings with arg0 matching
(`G_DBUS_SIGNAL_FLAGS_MATCH_ARG0_PATH`). In D-Bus 1.5, support was added
to the spec for also matching object paths. This got forgotten about and
was never added to GDBus, meaning that
`G_DBUS_SIGNAL_FLAGS_MATCH_ARG0_PATH` won’t match against a signal arg0
of type `o`.

Fix that, and add a unit test.

To do so, we need to add a new `g_dbus_message_get_arg0_path()` API to
complement the existing `g_dbus_message_get_arg0()` API. The approach of
letting `g_dbus_message_get_arg0()` return an object-path *or* a string
would not work, as it’s also called in the implementation of
`G_DBUS_SIGNAL_FLAGS_MATCH_ARG0_NAMESPACE`, which must only match
string-typed arg0 values and not object-path-typed ones.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3183