Such ID is not meant to be presented to the user, but to be an ID
which is stable over application development. Typically, this is
the gtkbuilder ID. Such an ID can then be used to identify a given
well-known accessible object independently from the current locale,
user-visible labelling, or dialog structure. This can then be used
for tailored screen reading (e.g. automatically announcing the
content of a well-known object), or for automatic regression testing
(to check the behavior of well-known objects). Uniqueness within
the window is thus desirable when returning a non-empty string.

This adds XAccessibleXContext2 to augment XAccessibleXContext with
getAccessibleId().  The VCL implementation just takes Window::get_id(),
i.e. the gtkbuilder id, which is enough for most use cases for now.

atk_object_wrapper_new then passes it to
atk_object_set_accessible_id() for AT-SPI technologies to pick it up.

Change-Id: Iea0ad08d036e539dbcfec1a9be026c8da8d17357
Reviewed-on: https://gerrit.libreoffice.org/71743Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins