[FEATURE] Base EXT: t3skin on twbs

Base TYPO3 CMS backend on twbs bootstrap. Brings bootstrap variables
adapted for TYPO3 CMS t3skin. There is no visual change.

This patch is the first step in a list of CSS refactorings.

Next steps:
Base more TYPO3 CMS backend widgets on twbs components.

Resolves: #61471
Releases: 6.3
Change-Id: I38df90a4925306a5aa8dd6ce5934ce5bd454a1db
Reviewed-on: http://review.typo3.org/32679
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>