Planned to be used in D1373.
Also added two BLI_str functions to remove trailing/leading whitespace in a string.
The custom string (custom as in: Defined in code but not on an ugly low level) is currently appended to the existing string, although it might be better to do it the other way around since the existing string changes its width continuously while transforming. The functions to remove whitespace wouldn't be needed anymore then, but think it would still be useful for some other cases?!