Track::init() must not call `add_processor` directly.
The track is not in the process graph, yet and hence various
calls (e.g. lock graph, update latency etc) are not applicable.
Furthermore ::add_midi_sidechain() calls IO::add_port()
which takes the process-lock.
The solution is to create the Triggerbox early on and let Route::init
deal with it like any other internal processors.
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