Conveyor Belt appearance when power is off (#2762)

* Conveyor Belt appearance when power is off

If the conveyor belt is running and the power turns off the belt stops pushing things but the animation continues. This changes it so the animation stops if there is no power and resumes when it returns.

* Change from using Initialize, to OnAdd and OnRemove

Resources/Prototypes/Entities/Constructible/Specific/Conveyor/conveyor.yml

@@ -27,6 +27,7 @@
     drawdepth: FloorObjects
   - type: SignalReceiver
     maxTransmitters: 1
+  - type: PowerReceiver
   - type: Conveyor
   - type: Appearance
     visuals:
@@ -34,7 +35,6 @@
       state_running: conveyor_started_cw
       state_stopped: conveyor_stopped_cw
       state_reversed: conveyor_started_cw_r
-  - type: PowerReceiver
   - type: Construction
     graph: ConveyorGraph
     node: entity