addprefix and addsuffix filter operators

Either I missed the mark completely on how to do this without the need for new addprefix and addsuffix operators or we had them really missing so far. In my Third Flow demo+docu wiki I'm using the `<<tabs>>` macro which only accepts a filter expression. I'm organizing the tab tiddlers with titles that start with the same prefix ("Third Flow") in my case and I'm putting only the remaining title part into the list field of the tiddler containing the `<<tabs>>` macro.  While we can remove prefixes and suffixes, we can't do the opposite. 

Added new filter operator for adding prefixes and suffixes to titles; these new filter operators are useful in contexts where only a filter expression is allowed and where macro processing isn't allowed. The filters complement the existing remove suffix and prefix filter operators. (So much for the "filter" in filter operator.)

Hopefully this all makes sense...