Make dbus_type_is_valid into public API

Make dbus_type_is_valid into public API

This is just as useful for bindings as dbus_signature_validate, and I
think it's a good design principle to say that anything checked in a
_dbus_return_if_fail should be something the caller could check
for themselves.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=20496
Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>