[FEATURE] Severity-filtering for FlashMessageQueue

This allows to fetch/clear only messages of a given
severity. FlashMessageContainer in TYPO3.Flow
offers similar functionality.

This is the basis for later extending the
FlashMessageViewHelper as well to only show messages
of a certain level.

Change-Id: Ibb84b5aa051be3a350cd97ae5c5058290748d4fe
Resolves: #7098
Releases: master
Reviewed-on: http://review.typo3.org/32941


Reviewed-by: Cedric Ziel <cedric@cedric-ziel.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Frederic Gaus <frederic.gaus@flagbit.de>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Daniel Maier <dani-maier@gmx.de>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>