less sources more flexible organized

At the moment there is no ability to put dependent graphic files (32px.png etc) to directory foreign to theme's compiled style sheet (style.css). There is the var `@image-path` but it located inside a main file, so there is no ability to point required images path without changing the source file.

Simple example, the jstree was installed via bower and the site's assets (css/js) includes jstree components (styles/scripts) then compiled with grunt. In that case we need to place the modified version of the jstree's `src/themes/default/style.less` file into the site's internal repo tree and support it.

This fix solves such an issue. Variables, mixins and main styles are located in separate files allows to fix some variables during building the site's assets.