Allow caching of data sent with events and requesting such data

Allow caching of data sent with events and requesting such data

Add atspi_event_listener_register_full, to request that particular data
be sent with an event, and cache the data for the duration of the event
callback.

This also extends the D-Bus protocol, adding an a{sv} to an event to send
this data along, but we will still read events without this array added for
backwards compatibility (and compatibilllity with QT-AT-SPI, which does not
send this data for now).

https://bugzilla.gnome.org/show_bug.cgi?id=708695