a11y: AT-SPI selection interface not supported

AT-SPI has a Selection interface that allows exposing selection information and changing selections via assistive technology.
Doc: https://lazka.github.io/pgi-docs/Atspi-2.0/classes/Selection.html

However, Qt currently does not implement this interface or provide any means for Qt applications to expose selections to assistive technology.

One use case where I ran into this is that the Orca screen reader on Linux does not announce selected cells in the Qt 6 based UI variant of LibreOffice.

I plan to suggest a potential implementation and will then also add a demo screencast for the LibreOffice case, but there are many more use cases and I think that some of Qt's own widgets would also profit from being extended to expose selection information to assistive technology.

Microsoft's UIA also has a means to handle selection that could be covered in addition to AT-SPI. (FWIW, IAccessible that is no longer used by Qt also supports selection via IAccessible::get_accSelection.)

Sample steps to reproduce

1) start LibreOffice Calc in the Qt 6 based variant of LibreOffice Writer

2) select a few cells
3) start Accerciser ( https://gitlab.gnome.org/GNOME/accerciser )
4) in Accerciser's treeview of LibreOffice's a11y object hierarchy, navigate to the a11y object for the spreadsheet
5) query the supported interfaces and explicitly try to query the Selection interface:

In [1]: acc.get_interfaces()
Out[1]: ['Accessible', 'Component', 'Table']
In [2]: acc.querySelection()
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
/usr/lib/python3/dist-packages/pyatspi/__init__.py in <module>
----> 1 acc.querySelection()

/usr/lib/python3/dist-packages/pyatspi/Accessibility.py in <lambda>(x)
    149 Atspi.Accessible.queryHypertext = lambda x: Hypertext(getInterface(Atspi.Accessible.get_hypertext_iface, x))
    150 Atspi.Accessible.queryImage = lambda x: Image(getInterface(Atspi.Accessible.get_image_iface, x))
--> 151 Atspi.Accessible.querySelection = lambda x: Selection(getInterface(Atspi.Accessible.get_selection_iface, x))
    152 Atspi.Accessible.queryTable = lambda x: Table(getInterface(Atspi.Accessible.get_table_iface, x))
    153 Atspi.Accessible.queryTableCell = lambda x: TableCell(getInterface(Atspi.Accessible.get_table_cell, x))

/usr/lib/python3/dist-packages/pyatspi/Accessibility.py in getInterface(func, obj)
     66         if ret:
     67                 return ret
---> 68         raise NotImplementedError
     69 
     70 def getEventType(event):

NotImplementedError: 

Actual result:

The AT-SPI selection interface is not supported and there seems to be no way for LibreOffice to fix that without adding corresponding API to the Qt library.

Expected result:

There should be a way to expose the AT-SPI selection interface in Qt applications (and ideally, some of the Qt widgets should expose it, too).