[BUGFIX] colPos_list sets access to edit areas

The property mod.SHARED.colPos_list used to determine which columns
were visible in the Page module. With backend layouts it will only
determine which edit areas in the backend layout are accessible for
editing.

Resolves: #39967
Releases: 6.3, 6.2
Change-Id: Iba65ce84adf025802d1b0ebda00dff4d6299a06a
Reviewed-on: http://review.typo3.org/32615
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>