motion_queuing: Add new module to help with motion queues and flushing

Create a new module to assist with host management of motion queues.
Register all MCU_stepper objects with this module and use the module
for step generation.

All steppers will now automatically generate steps whenever
toolhead._advance_flush_time() is invoked.  It is no longer necessary
for callers to individually call stepper.generate_steps().

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/kinematics/extruder.py

@@ -39,8 +39,6 @@ class ExtruderStepper:
                                    self.name, self.cmd_SYNC_EXTRUDER_MOTION,
                                    desc=self.cmd_SYNC_EXTRUDER_MOTION_help)
     def _handle_connect(self):
-        toolhead = self.printer.lookup_object('toolhead')
-        toolhead.register_step_generator(self.stepper.generate_steps)
         self._set_pressure_advance(self.config_pa, self.config_smooth_time)
     def get_status(self, eventtime):
         return {'pressure_advance': self.pressure_advance,