GDBus: Add GDBusSignalFlags and use it in g_dbus_connection_signal_subscribe()

This is currently unused but will probably be useful in the
future. For example, we could have a _ARG0_IS_PATH to specify that
arg0 should be used for arg0path.

This commit breaks API and ABI. Users of
g_dbus_connection_signal_subscribe() will need to port to this new
version.

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