[AT-SPI] Add ActorAccessible

[AT-SPI] Add ActorAccessible

This change adds a new ActorAccessible class as a common ancestor for
both AdaptorAccessible and ControlAccessible. This will allow to reduce
code duplication between dali-adaptor and dali-toolkit. There are no
significant changes to methods moved from AdaptorAccessible to
ActorAccessible, as they will be reworked in a later change.

Change-Id: I61fb8c6610e761cd8d49f8cf67f61af35cfe8dda