Implement pad added/removed callback
Implement pad added/removed callback

Change-Id: I7bb4d0e71e0b6b222afc6b73528464eb461e2d9e

Kurento/kms-elements,Kurento/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,TribeMedia/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,Kurento/kms-elements,shelsonjava/kms-elements