Prevents plastanium conveyor unload exploit (#4098)

* Prevent unloaders from unloading plast loading dock

* Use overridable boolean method unloadable instead

* unloadable -> canUnload

core/src/mindustry/entities/comp/BuildingComp.java

@@ -384,6 +384,10 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
     //endregion
     //region handler methods
 
+    public boolean canUnload(){
+        return block.unloadable;
+    }
+
     /** Called when an unloader takes an item. */
     public void itemTaken(Item item){