When to use strfwd.h

[Pavel Sanda]

Hi,

in GuiToolbar.h we include strfwd.h, clearly because
we use std::string there (as introduced by Riki in
cc4bfc7f04d).

However in many different .h files we include <string>
for the same purpouse. I suspect we should be consistent
but I am not sure what is correct.

Is the only purpose of strfwd to avoid lengthy standard
string/stream headers? If so should we replace <string>
includes in rest of headers by strfwd?

Pavel