Use a Sequence of XAccessible rather than its base
interface XInterface for AccessibleRelation's TargetSet.
As the targets are accessible objects as well,
anything other than XAccessible doesn't make much sense.
Using XAccessible right away makes that clearer and avoids
the need to query the XAccessible interface.
(The winaccessibility bridge was already using
`static_cast`, relying on the fact that the objects
are XAccessibles.)
The a11y UNO API is not published, so an API change
should be unproblematic.
Change-Id: I7f08e98d1ec303d5343d9a7954187cdd71495ebc
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166586
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins
