added transferFrom() to Inventory

fxgl-gameplay/src/main/kotlin/com/almasb/fxgl/inventory/Inventory.kt

@@ -193,6 +193,27 @@ class Inventory<T>(
 
         return isOK
     }
+
+    /**
+     * Transfer [item] with [quantity] amount from [other] to this inventory.
+     *
+     * @return true if operation was successful
+     */
+    @JvmOverloads fun transferFrom(other: Inventory<T>, item: T, quantity: Int = 1): Boolean {
+        if (isFull)
+            return false
+
+        if (!other.hasItem(item))
+            return false
+
+        if (other.getItemQuantity(item) < quantity)
+            return false
+
+        add(item, quantity = quantity)
+        other.incrementQuantity(item, -quantity)
+
+        return true
+    }
 }
 
 class ItemData<T> internal constructor(var userItem: T) {

fxgl-gameplay/src/test/kotlin/com/almasb/fxgl/inventory/InventoryTest.kt

@@ -238,4 +238,24 @@ class InventoryTest {
 
         assertTrue(inventory.incrementQuantity("Hello", -10))
     }
+
+    @Test
+    fun `Transfer from another inventory`() {
+        inventory = Inventory(5)
+
+        inventory.add("Hello", quantity = 5)
+
+        val other = Inventory<String>(6)
+
+        var result = other.transferFrom(inventory, "Hello", 4)
+
+        assertTrue(result)
+        assertThat(other.getItemQuantity("Hello"), `is`(4))
+        assertThat(inventory.getItemQuantity("Hello"), `is`(1))
+
+        // quantity only 1, so cannot transfer
+        result = other.transferFrom(inventory, "Hello", 4)
+
+        assertFalse(result)
+    }
 }