Upload styles

Feature #9239

I've polished this feature a bit, rebased the branch upload_styles and I think this feature is ready for a pull request. During the rebase I might have included some files that shouldn't make it in there, please beware of those.

Right now the styles are uploaded to darktablestyles.sourceforge.net, but this can easily be set in src/gui/styles_upload.c:41.

There is at the moment no visual feedback on thumbnails being rendered, so you might start to wonder why no thumbs are showing up. Best thing would probably be to have a text like "rendering" inside the thumbnail boxes or something.

Please let me know if you have further questions!

Robert