provisional changes to speed up solo changes to large numbers of routes.

provisional changes to speed up solo changes to large numbers of routes.

Moves global update of solo state and emission of Session::SoloChanged to
a single point after 1 to N solo controls are changed.

Also avoid unnecessarily emitted Activated() signal for listen controls,
though Process::{activate,deactive}() should probably be redesigned to
avoid this in a "deeper" way