Rename textbox to textboxes and textbox line to lines

It's really dumb that these array names aren't plural when they should
be, because they contain more than one thing.

desktop_version/src/Graphics.h

@@ -300,7 +300,7 @@ public:
     bool trinketcolset;
     int trinketr, trinketg, trinketb;
 
-    std::vector <textboxclass> textbox;
+    std::vector <textboxclass> textboxes;
 
     bool showcutscenebars;
     int cutscenebarspos;