qt a11y: Clear XAccessible reference when disposing

The `XEventListener::disposing` doc
(in `./udkapi/com/sun/star/lang/XEventListener.idl`) says:

> /** gets called when the broadcaster is about to be disposed.
>
>     <p>All listeners and all other objects, which reference the
>     broadcaster should release the reference to the source.
>     No method should be invoked anymore on this object (
>     including XComponent::removeEventListener() ).
>     </p>
>
>     <p>This method is called for every listener registration
>     of derived listener interfaced, not only for registrations
>     at XComponent. </p>
>  */

Therefore, clear the `XAccessible` reference held in
the associated `QtAccessibleWidget`
in `QtAccessibleEventListener::disposing`.

This also implies that `QtAccessibleWidget::isValid()`
(overriding the `QAccessibleInterface` method)
will return `false` from that point on.

Change-Id: I077dffe6ca3e887707d1f578e947ccf3c2c2a492
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/138364
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

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