Make dialog window modal

Fixes #408

Author: alexbatalov

src/game_dialog.cc

@@ -4458,7 +4458,7 @@ static int talk_to_create_background_window()
         GAME_DIALOG_WINDOW_WIDTH,
         GAME_DIALOG_WINDOW_HEIGHT,
         256,
-        WINDOW_DONT_MOVE_TOP);
+        WINDOW_DONT_MOVE_TOP | WINDOW_MODAL);
 
     if (gGameDialogBackgroundWindow != -1) {
         return 0;