VOXEL: switch from palette-XXX.png palettes to gimp palettes

[mgerhardy]

This allows us to display more details - like a name, maybe author and/or download link

See issue #174 for e.g. lospec support - it would be great to have the direct link in the palette panel and maybe even a description in the selection dialog

[mgerhardy]

• https://robert-96.github.io/gimp-color-palettes/
• https://lospec.com/palette-list