D-Bus: keep type information in D-Bus events

D-Bus: keep type information in D-Bus events

* doc/misc/dbus.texi (Errors and Events):
* etc/NEWS: D-Bus events keep the type information of their arguments.

* lisp/net/dbus.el (dbus-check-event): Fix docstring.
(dbus-delete-types, dbus-flatten-types): New defuns.
(dbus-handle-event, dbus-register-property, dbus-property-handler):
Handle type information.
(dbus-set-property): Fix thinko.

* src/dbusbind.c (XD_BASIC_DBUS_TYPE): Simplify.
(xd_dbus_type_to_symbol): New function.
(xd_retrieve_arg): Return type information for the arguments.
(xd_read_message_1):  Return type information for the error name.
(dbus-registered-objects-table): Fix docstring.