Extend search and replace to work with white space characters

It should be possible to

• enter multiple multiple spaces directly after each other in the search or in the replace field and thus search for white space tags that represent this and replace them
• Usage of \n for a line break, \t for a tab and \s for all whitespace in the pcre regex syntax should be supported.

Caution: if there is a white space tag that represents 3 spaces and before it there is a forth real space in the segment and a user searches for 2 spaces, this should be a match. A replace action should then change the white space tag accordingly. Analogous for the search with regular expressions for white space characters.