[FEATURE] #5100 Add convenience function for accessing sent SwiftMails

Change-Id: I2806e686f44b4252af3146ff99cae1f0317f0d73
Reviewed-on: http://review.typo3.org/32485
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>