new field: engine_group (Bus/Track), multiengine support

Effects.p: fix tests by using ::this_op() and ::valid_engine_setup() instead of intended import name.
port setting for default engine
engine commands: new_engine, select_engine, set_track_engine_group, set_bus_engine_group
Generate chainsetup with -G:$this_engine->name,$this_engine->jack_operation_mode