a11y: Report QAccessible::Help as HelpText AT-SPI property

Qt has a QAccessible::Help  enum value for text. Quoting https://doc.qt.io/qt-6/qaccessible.html#Text-enum : "A longer text giving information about how to use the object."

However, this is currently not bridged to/reported on the AT-SPI platform level on Linux.

AT-SPI2 got a HelpText attribute a few months ago:

https://gitlab.gnome.org/GNOME/at-spi2-core/-/commit/8f1c70155b4132de2c567b3034e4f242e0450df5

Steps to reproduce:

0) run a system that has libatspi version >= 2.52 (e.g. current Fedora Rawhide; for the API added in the above -mentioned at-spi2-core commit to be available in step 4)

1) build and run the attached sample program

2) start Accerciser ( https://gitlab.gnome.org/GNOME/accerciser )

3) in Accerciser's treeview of the sample application's a11y tree, select the button

4) query the HelpText accessible attribute via Accerciser's IPython console:

In [1]: acc.get_help_text()

Actual result:

No help text gets returned, even though the WhatsThis text was set for the button, and that gets used as QAccessible::Help text Qt-internally:

In [1]: acc.get_help_text()
In [2]: acc.get_help_text() == None
Out[2]: True 

Expected result:

The WhatsThis text should be reported as help text. Output with a local change in place (I will submit that one to Gerrit):

In [3]: acc.get_help_text()
Out[3]: 'Some WhatsThis text that gets used as the accessible help text'