Update ImageLayer specification

[nightm4re94]

<imagelayer>

• [ ] class: The class of the image layer (since 1.9, defaults to “”).
• [ ] repeatx: Whether the image drawn by this layer is repeated along the X axis. (since Tiled 1.8)
• [ ] repeaty: Whether the image drawn by this layer is repeated along the Y axis. (since Tiled 1.8)