GUI: Using the KGameDifficulty class to manage difficulty levels in games now adds automatically a combobox in the status bar to display the current difficulty level and to let the player change it.

To get this new feature, nothing to change in the code of the games that already use KGameDifficulty. Main idea was to do like in bovo. (Note that bovo don't use KGameDifficulty yet).

For examples, see Kmines, KBlackBox (and soon Kenolaba).

Feedback is as always welcomed.