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.cpp

@@ -27,6 +27,7 @@ textboxclass::textboxclass(void)
     large = false;
 
     print_flags = PR_FONT_LEVEL;
+    fill_buttons = false;
 }
 
 void textboxclass::centerx(void)