explanatory comment
don't confuse arm with i686
change inheritance so that an AudioBackend IS-A PortEngine

This allows a derived (concrete) implementation to share information (e.g. sample rate, buffer size)
between the audio backend side of things and the port management side of things.
update JACK backend to use new inheritance structure for AudioBackend
use visibility control on JACK audio backend, so that we can import this change into windows branch
Merge branch 'master--arm' of https://github.com/nphilipp/ardour