Fix prop type validation warning in SpriteSelectorItem

In ./src/containers/costume-tab.jsx:271 the SpriteSelctorItem
`dragPayload` prop is set to be an object. To supress the prop type
warning, I've added `propTypes.object` to the array of allowed types.

Author: chrislo

src/containers/sprite-selector-item.jsx

@@ -151,7 +151,11 @@ SpriteSelectorItem.propTypes = {
     asset: PropTypes.instanceOf(storage.Asset),
     costumeURL: PropTypes.string,
     dispatchSetHoveredSprite: PropTypes.func.isRequired,
-    dragPayload: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
+    dragPayload: PropTypes.oneOfType([
+        PropTypes.string,
+        PropTypes.number,
+        PropTypes.object
+    ]),
     dragType: PropTypes.string,
     dragging: PropTypes.bool,
     id: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),