Ability to import palettes from file

[ghost]

It would be really nice to have the ability to import (and maybe export) palettes from some common file formats!

The GIMP GPL format is very simple, text-based and is thoroughly documented in their Developer Guide.

The JASC PAL format also is just a text-based list of number values and it's even more barebones than GPL. I found some reference here.

There's also the Photoshop ASE format, but I couldn't find a lot about it :c

I'm still learning Godot, but I could try opening a PR for this feature if someone finds it useful!

[ghost]

I actually ended up working on it anyway as an exercise, I have a rough working implementation but may need some pointers on how to organise the functions to better fit the project style