[BUGFIX] Update sys_refindex to reflect img tags in RTE content

Problem: sys_refindex should be updated when an image is inserted or
removed from RTE content
Solution: Add rtehtmlarea_images softref key

Change-Id: I40d504adf3d51e55fd271d01656e9db3f285d7ad
Releases: 6.0
Resolves: #36789
Reviewed-on: http://review.typo3.org/10940
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland