_atspi_dbus_set_interfaces: split internals into a demarshaler and a translator
_atspi_dbus_set_interfaces: split internals into a demarshaler and a translator

First we demarshal the DBusMessageIter into an InterfaceNames struct.

Then we convert the InterfaceNames to a bitmask suitable for the field
AtspiAccessible.interfaces.

This will make it easier to replace libdbus with gdbus.

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core