New class: PreInsertDryTrack, like SlaveTrack, but using own sends
Use SlaveTrack to implement ::PrefaderInsert wet send path.

This avoids multiple prefader inserts clobbering the same
graph, for example:

	jack_client_in -> jack_client_out

This requires adding an auxiliary track, for example:

	jack_client_in -> sax-wet-send -> jack_client_out
PrefaderInsert add_path() method: override mono_to_stereo in vertex attributes
Was edge attribute, which didn't override.
Rewrite chainsetup generation description, move to ::ChainSetup
Revert "New class: PreInsertDryTrack, like SlaveTrack, but using own sends"

This reverts commit 356342d6acd43b56af1747845dc925f29148f04a.

We're not using this approach. Connections are instead made by
::PrefaderInsert's add_paths() method.
Merge branch 'insert-graph'