a11y AT-SPI: "GetCaption" result (from AT-SPI table interface) has incorrect D-Bus type

Curently, Qt returns the result of the "GetCaption" method of the AT-SPI table interface in an incorrect D-Bus type, which means it can't be processed properly by AT (assistive technology).

Sample steps to reproduce, with qtbase dev branch as of commit fa4db87706c39c62b94017a724159c55ecc0053f follow.

Steps to reproduce:

1. build and run the "tabdialog" example from qtbase:

   $ qtbase/examples/widgets/dialogs/tabdialog/tabdialog
   

1. run the attached sample pyatspi script that finds an object with AT-SPI table role and calls the "GetCaption" method on it.

Alternative:

1. build and run the sample program as mentioned above
2. start Accerciser ( https://gitlab.gnome.org/GNOME/accerciser ) from a terminal (to see the output)
3. in Accerciser, navigate to the a11y object for the QListWidget in the "Applications" tab of the sample application.

(QListWidget implements QAccessibleTableInterface, which maps to AT-SPI's table interface, and Accerciser automatically queries the attributes when the object is clicked.)

Actual result:

The script (or Accerciser, for the alternative) print something like this on stderr:

(accerciser:181933): dbind-WARNING **: 11:50:36.394: atspi_dbus_get_property: expected a variant when fetching Caption from interface org.a11y.atspi.Table; got (so)

Expected result:

No warning due to incorrect types should appear in the output of the script/Accerciser.