[FEATURE] Refactor EXT: t3skin CSS to Less & variables

Make use of Less hierarchy and constraints. Dissolves "visual" and
"structure" folders in order to use Less variables for "Visual"
layout at a later point.

This patch:
+ moves Less files
+ merges visual and structure sub-folders
+ merges several CSS classes into hierarchical format.

Patch explicitly does not yet:
+ refactor CSS definitions
+ removes duplicated CSS definitions
+ make use of Bootstrap where applicable.

Layout in the backend before and after this patch does not change.

Resolves: #62042
Releases: master
Change-Id: Ic9045a565c3a81808431c217379ffb375807c071
Reviewed-on: http://review.typo3.org/33170
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>