Refactor text boxes to not use the 'active' system

This removes the variables graphics.ntextbox, as well as removing
'active' from each text box object. Thus, all text boxes are really
real, and you don't have to check its 'active' variable.

desktop_version/src/Textbox.h

@@ -41,7 +41,6 @@ public:
     int r,g,b;
     int tr,tg,tb;
     SDL_Rect textrect;
-    bool active;
     int timer;
 
     float tl;