manual_stepper: Implement "drip moves" for manual stepper STOP_ON_ENDSTOP

Currently, `MANUAL_STEPPER STOP_ON_ENDSTOP=1` type commands will move
until hitting the endstop, but it will still always consume the total
amount of move time.  That is, following moves can't be started until
the total possible time of the homing move is completed.

Implement "drip moves" so that the code only schedules the movement in
small segments.  This allows following movements to be scheduled
without a significant delay.

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

klippy/toolhead.py

@@ -503,7 +503,7 @@ class ToolHead:
     def get_extruder(self):
         return self.extruder
     # Homing "drip move" handling
-    def drip_update_time(self, next_print_time, drip_completion):
+    def drip_update_time(self, next_print_time, drip_completion, addstepper=()):
         # Transition from "NeedPrime"/"Priming"/main state to "Drip" state
         self.special_queuing_state = "Drip"
         self.need_check_pause = self.reactor.NEVER
@@ -526,6 +526,8 @@ class ToolHead:
             npt = min(self.print_time + DRIP_SEGMENT_TIME, next_print_time)
             self.note_mcu_movequeue_activity(npt + self.kin_flush_delay,
                                              set_step_gen_time=True)
+            for stepper in addstepper:
+                stepper.generate_steps(npt)
             self._advance_move_time(npt)
         # Exit "Drip" state
         self.reactor.update_timer(self.flush_timer, self.reactor.NOW)