Implemented item dragging and transferring

core/src/io/anuke/mindustry/resource/CarryItem.java

@@ -0,0 +1,4 @@
+package io.anuke.mindustry.resource;
+
+public interface CarryItem {
+}

core/src/io/anuke/mindustry/resource/ItemStack.java

@@ -1,6 +1,6 @@
 package io.anuke.mindustry.resource;
 
-public class ItemStack{
+public class ItemStack implements CarryItem{
 	public Item item;
 	public int amount;
 	

core/src/io/anuke/mindustry/resource/LiquidStack.java

@@ -1,6 +1,6 @@
 package io.anuke.mindustry.resource;
 
-public class LiquidStack {
+public class LiquidStack implements CarryItem{
     public Liquid liquid;
     public float amount;