Change font::len text argument from std::string to const char*

See the previous two commits, a lot of the time we don't need
std::string objects to be passed to these functions because we already
have C strings.

   Commit 1/3: font::print_wrap
   Commit 2/3: font::print
-> Commit 3/3: font::len

desktop_version/src/Textbox.cpp

@@ -106,7 +106,7 @@ void textboxclass::resize(void)
     int max = 0;
     for (size_t iter = 0; iter < lines.size(); iter++)
     {
-        int len = font::len(print_flags, lines[iter]);
+        int len = font::len(print_flags, lines[iter].c_str());
         if (len > max) max = len;
     }