Use const std::string& where possible in function params

If it's at all possible to use `const std::string&` when passing
`std::string`s around, then we use it. This is to limit the amount of
memory usage as a result of the frequent use of `std::string`s, so the
game no longer unnecessarily copies strings when it doesn't need to.

desktop_version/src/Textbox.cpp

@@ -107,7 +107,7 @@ void textboxclass::resize(void)
     h = (line.size() + 2) * 8;
 }
 
-void textboxclass::addline(std::string t)
+void textboxclass::addline(const std::string& t)
 {
     line.push_back(t);
     resize();