[ATSPI] Fix for DefaultLabel

[ATSPI] Fix for DefaultLabel

Previously the call to GetDefaultLabelInfo() was delegated by bridge to
the corresponding Accessible object which computed the reply locally (by default returned self).
However default labels should be managed globally within given application.

This patch:

* replaces existing API AddPopup()/RemovePopup() by RegisterDefaultLabel()/UnregisterDefaultLabel()
  which has wider applicability (not only for popups)
* changes the logic of GetDefaultLabelInfo() to compute default label object globally

Related patches:
https://review.tizen.org/gerrit/#/c/platform/core/uifw/dali-toolkit/+/267323/
https://review.tizen.org/gerrit/#/c/platform/core/uifw/dali-csharp-binder/+/267375/

Change-Id: I68ccaf55835109139ba00e2204fcdfcf2627d403