So far, there was an internal copy of the IAccessible2
IDL file added in 2013, and never updated since then
(except for cosmetic changes like formatting and typo
fixes), and therefore it e.g. doesn't have the
`IA2_ROLE_BLOCK_QUOTE` role needed for tdf#135586.

Drop the outdated internal copy and integrate an up to
date upstream version (current git master) instead.

Instead of duplicating things in an internal copy,
switch to using the usual mechanism for externals.

The IAccessible2 source contains separate .idl files
for the different interfaces, while the previous
interal copy was already the merged IDL file.

Call the IAccessible2 `./concatidl.sh` script to generate
the merged `ia2_api_all.idl`
(s. external/IAccessible2/ExternalProject_IAccessible2.mk)
and adapt the winaccessibility make files to use that
version instead of the internal copy.
(Add a dependency to the target that generates
`ia2_api_all.idl`.)

The tarball was generated from the master branch of the
IAccessible2 git repo [1] as of

    commit 3d8c7f0b833453f761ded6b12d8be431507bfe0b (HEAD -> master, origin/master, origin/HEAD, mygithub/master)
    Author: Michael Weghorn <m.weghorn@posteo.de>
    Date:   Fri Oct 13 04:16:03 2023 +0300

        Update no longer working links (#26)

        * Update link to Linux Foundation Trademark Policy
        * Update link to Object Attributes specification

        Signed-off-by: Michael Weghorn <m.weghorn@posteo.de>

using this command:

    $ git archive -o ../IAccessible2-1.3+git20231013.3d8c7f0.tar.gz --prefix=IAccessible2-1.3+git20231013.3d8c7f0/ master

Andrea Gelmini's typo fixes have been upstreamed in [2].

[1] https://github.com/LinuxA11y/IAccessible2
[2] https://github.com/LinuxA11y/IAccessible2/pull/21

Change-Id: I9f19ff55694da5d3a9a8750be8de387cbf6da785
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/158427
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>