toolkit a11y: Use OAccessibleComponentHelper for grid control

toolkit a11y: Use OAccessibleComponentHelper for grid control

Simplify AccessibleGridControlBase by subclassing the
comphelper::OAccessibleComponentHelper helper class
that already provides much of the logic.

Unless I'm missing anything, the corresponding custom
implementations in AccessibleGridControlBase were
equivalent (and partly looked copy-pasted).

Notes:

* XAccessibleEventBroadcaster logic is fully implemented
  in OAccessibleComponentHelper, so drop the custom logic.

* In order to implement XAccessibleComponent methods,
  only OAccessibleComponentHelper::getBounds needs
  to be overriden. The logic is already implemented
  in AccessibleGridControlBase::implGetBoundingBox,
  just need to convert from tools::Rectangle to
  css::awt::Rectangle in addition.
  Drop all of the other methods related to location/bounds
  handling. The AccessibleGridControlBase logic takes
  care of that now.

* Drop AccessibleGridControlBase::ensureIsAlive and
  use OAccessibleComponentHelper::ensureAlive instead,
  which has the same logic.

In a quick test using sample document attachment 198647
from tdf#164783 with the qt6 VCL plugin and Accerciser
and the Orca screen reader, no differences in behavior
were observed with this change in place.

Change-Id: I90099dd5426db093260175b2c3d09db548e05c33
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/180723
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>