a11y: QFrame reports incorrect top-level role on Linux (AT-SPI2)

Using QFrame in an application results in wrong accessible roles being reported on the accessibility layer (AT-SPI2) on Linux: It is reported with role ATSPI_ROLE_FRAME, which means [1]:

> A top level window with a title bar, border, menubar, etc.

However, that does not fit for QFrame, which is semantically rather a container for other widgets. Having a non-top-level widget report the AT-SPI framerole  is misleading, and could e.g. confuse screen readers.

Steps to reproduce:

1) compile and run the attached sample program

2) start Accerciser

3) check the accessibility hierarchy of the app in Accerciser

Actual result:

Besides the QMainWindow which is (expectedly) reported as a "frame" on AT-SPI layer, there are more children with the frame role. (s.a. attached screenshot)

Expected result:

Only the QMainWindow which is actually a top-level window/frame should report that role on AT-SPI2 level.

[1] https://docs.gtk.org/atspi2/enum.Role.html#frame