Export groups as folders

[skjorn]

Make an option to export the layer tree as a file tree, where layer groups would correspond to folders.

(Clean up old file tree with the same root folder name? Restart file name indices in sub-folders? Apply file name prefix to folders as well? There's a lot of details to figure out...)

[Grawl]

This will be awesome.

[m-schou]

Would incredible usefull. If you want money for it i'll pay.

[Bahanix]

Available in PR #60 👍

[skjorn]

Done.