selectablemixer: Add skelleton class

Change-Id: Id6ba2db599564a2ad513712810da7c5e4ee67ebc
selectablemixer: Handle/unhandle ports

Change-Id: I3ae740512b8ed626e047ca0a2066de6a3cc2f5e0
selectablemixer: Connect video ports

Change-Id: I989213094addfb7a33f473f569a5aeab17473034