Export in xcf format

[k-aito]

Hello, after the recommendation of zyddnys to take a look at this project if I want to be able to make modifications on the fly, and he got it right.

I looked a bit and I really like how it's done, I only think that it would be nice to be able to output a xcf if there is a need to make other modification with Gimp for example when inpainting or removing is not enough.

I noticed #50 and I wouldn't mind a PSD export if the text stays editable, but I'm on Linux, so I will more be interested in Gimp itself.

No promise, I will try to take a look and do a PR based on what I see in zyddnys code and to adapt it here, but if someone is faster or wants to do it don't wait for me.

Cheers