[BUGFIX] Use trimExplode() for pageOverlayFields

Currently explode() is used for pageOverLayFields which requires a list
without any whitespaces. Using t3lib_div::trimExplode() improves
the usability for integrators and reduces possibilities of not working
configurations

Change-Id: I7c6c3412c1a1a250aac273169060dbd4aaf4e1ca
Resolves: #28916
Releases: 4.4, 4.5, 4.6
Reviewed-on: http://review.typo3.org/4359
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers