Interpolate text box alpha

This makes text boxes fade in and out pretty smoothly.

This requires that the textboxclass::setcol() be in Graphics::drawgui(),
so now it's moved there.

Text box fading is only really noticeable if you're playing in slowmode.

desktop_version/src/Textbox.h

@@ -40,6 +40,7 @@ public:
     int timer;
 
     float tl;
+    float prev_tl;
     int tm;
 
     int max;