This makes is possible to generate a valid D-Bus XML from a class that has
e.g.  internal slots with pointer parameters by explicitly marking all D-Bus
exports with Q_SCRIPTABLE and running qdbuscpp2xml with the '-a' switch.

Change-Id: Iab32a0a7f46f2516f6385808bbf35b26c7708b1a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>