Reorganization of atk-main.css

1) Made a backup folder atk-main-css-backup and moved everything there. This should be removed after a while when more testing is done.
2) Split whole atk-main stylesheet in multiple LESS files (sub-folder /less).
3) Fixed some bugs in stylesheet (double declarations etc.)
4) Made some ATK variables dependant from JUI variables. There more work should be done by ATK designer. For example to generate gradient high and low colors from a single JUI color defined in jui-variables.less etc.
5) All of this is slightly inspired by Bootstrap template structure. Actually it was before too as far as I can tell, but strangely everything was put together in a big LESSfile at the end. That way it was harder to make your own adjustments to stylesheet (for example to just simply change some less variables) and recompile. Now you can make changes only where needed and recompile to single CSS for your project.

All of this definitely need some more testing. I have done some, but will not pretend that I tested everything :)

Also there more work should be done to split ATK template by components in multiple LESS files even better. Some declarations was left in misc.less and maybe some are not added to correct component file (form, grid).