Now that the accessibility module no longer
depends on toolkit and svtools and there's
therefore no more cyclic dependency, merge it
into the vcl library, where the vcl widgets are located,
for which the classes in the accessibility
module (primarily) provide the a11y classes.

Initially, take over the existing UNO service
"com.sun.star.accessibility.comp.GetStandardAccessibleFactoryService"
to vcl as is. It used to be necessary to break the cyclic
dependency between the vcl, svtools, toolkit and
accessibility module. (vcl is the lowest and
accessibility used to be furthest up in the dependency
chain; yet vcl needed it to create the objects
providing the XAccessible/XAccessibleContext
for its widgets.)

Further simplification can happen in upcoming commits.

Change-Id: Ib46c87446dc9121d3b8e735e0e5a40594da73cc5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/178647
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>