assets: remove all _CSS definitions. All css files renamed with .less extension
style: use less syntax, added variables.less to overrides definitions from included packages
fix CSS causing trouble when parsed as a less file: removed rules for IE5.5 support in datatable theme
assets: preserver order of css import; fix wrong sources references in css source map file.

Now all CSS are imported as LESS content, so that url() values are properly rewritten; as a bonus order of inclusion is preserved (not the case when importing as CSS content.