GIMP plugin: Add encode options dialog

[petrdiblik]

It would be great if the plugin showed a dialog with encode options before actual saving.
The options could be: Distance, Quality, Effort, Progressive (which corresponds with cjxl -h).
A brief explanation of the options could be there as well.

(The plugin can't easily decide itself whether it should use lossy/lossless in the same way as cjxl because the input are pixels, not image formats.)