Add textbuttons() script command, make Violet's ENTER dialogue dynamic

Violet's dialogue now looks like this:

squeak(purple)
text(purple,0,0,2)
Remember that you can press {b_map}
to check where you are on the map!
position(purple,above)
textbuttons()
speak_active

The new textbuttons() command sets the next textbox to replace {b_map}
with the map button, and {b_int} with the interact button. The
remaining keys would be added as soon as they need to be added to
ActionSets.h as well.

desktop_version/src/Textbox.h

@@ -52,6 +52,7 @@ public:
     bool large;
 
     uint32_t print_flags;
+    bool fill_buttons;
 };
 
 #endif /* TEXTBOX_H */