[TASK] Move jquery.clearable.js to ext:backend

This also makes jquery.clearable a solid standalone
AMD module.

Resolves: #66161
Releases: master
Change-Id: Id0f01b36c7e4d5050a9b3099db44c760c2234e10
Reviewed-on: http://review.typo3.org/38360


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>