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/Graphics.h

@@ -111,6 +111,8 @@ public:
 
     void textboxprintflags(uint32_t flags);
 
+    void textboxbuttons(void);
+
     void textboxcommsrelay(void);
 
     void textboxadjust(void);