[BUGFIX] IMG_RESOURCE not capable of sys_file_reference

The file.import function can't handle file relations. Sometimes
you have to deal with uids of sys_file_references instead of
sys_file uids. One prominent case is import.data = levelmedia
This patch adds an flag to IMG_RESOURCE with that, a given uid
is interpreted as reference instead of file uid.

Change-Id: I4699d9e38b9a809fd194d33344678a64168d4300
Fixes: #43101
Releases: 6.0
Documentation: #43283
Reviewed-on: http://review.typo3.org/16584
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Stefan Neufeind
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel