[BUGFIX] Page module content element min-width (CSS)

Adds min-width to content elements in page backend module
to fix problem with line breaking icons.

Change-Id: Id66622c91433782e0813b6fb20097fa7a0c48793
Fixes: #35962
Releases: 6.0
Reviewed-on: http://review.typo3.org/10390
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Stefan Galinski
Tested-by: Stefan Galinski