Implement category change signal notifications

:Release Notes:
Implement signal subscription and propagation.

:Detailed Notes:

:Testing Performed:
make test

:QA Notes:

:Issues Addressed:
[BHV-1252] Add 2-step ls2 bus registration APIs

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: Ief36d6cd2e415340ebf4096b9dd0fb5d64f3d300
Reviewed-on: https://g2g.palm.com/5334
Reviewed-by: DCO Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>