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.h

@@ -25,7 +25,7 @@ public:
 
     void resize(void);
 
-    void addline(std::string t);
+    void addline(const std::string& t);
 public:
     //Fundamentals
     std::vector<std::string> line;