[BUGFIX] Leading and trailing blanks can't be edited by the RTE

This patch will:
1. collapse repeating spaces into a single one;
2. replace leading and trailing spaces with non-breaking spaces;
3. replace any space following > and any space preceding < by a
   non-breaking space.

Change-Id: I06a8996f23e050e2e30cc2548c9c1bbafd5b41f0
Resolves: #16217
Releases: 4.5, 4.6
Reviewed-on: http://review.typo3.org/4758
Reviewed-by: Björn Pedersen
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers