Gave cv::String appropriate += operators.

Note that since String is a reference to an immutable string,
this doesn't actually change the string; it just replaces *this
with a reference to the concatenated string.
Replaced most of the instances of "s = s +" with "s +=".

This resynchronizes master code with 2.4. I say "most", because I might
have missed some due to different spacing; I'll deal with those
instances later if I find them.
Merge pull request #1859 from SpecLad:string-plus-equals