[BUGFIX] Make alphabetic filter work with umlauts

Changes the regular expression used to validate the string into a
unicode-aware character-class and adds two testcases for this.

Change-Id: I247437d9e722c4656ddab0dd2e2ef6a960837cff
Fixes: #35296
Releases: 4.5, 4.7, 6.0
Reviewed-on: http://review.typo3.org/10066
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf