Some points to improve about #532 for the next beta:

• Add support for RGB reference layer on indexed images
• Improve UI to resize reference layer cels (now we can resize reference layers moving the mouse to the bottom-right corner of the ref cel) (UX: Reference layers should either be transformable, or show handles when selected #3380)
• Add possibility to link an external file as reference layer (so we don't save the reference layer each time we File > Save the sprite)