[FEATURE] Signal/Slot in ImageResizer before reindexing

This signal lets you post-process the image after it has been resized but before
the original file is actually replaced. Based on new dimensions or file size, you
could choose to process it alternately with an external tool.

Change-Id: Iaadfca543fd7198c484828d091870bd0b3bc6a03
Resolves: #54672
Reviewed-on: https://review.typo3.org/28033
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers