new API for route solo/mute state mgmt

new API for route solo/mute state mgmt

Route now calls back into Session when solo/mute/listen state changes. All other interested
parties must use the Route::{solo,mute,...}_control()->Changed() to be notified of changes.

The Session requires more information than the Changed signal can provide, in order to
propagate solo/mute changes across the entire Session correctly.

Note that this uses an experimental use of CRTP to isolate a public API within Session