Implement mnemonics

Here is what I came up with based on the [discussion on Google Groups](https://groups.google.com/forum/#!topic/xwt-list/Vj0vq2JqeYY).

I have just implemented this for MenuItems and Buttons so far. There is a new property called UseMnemonic that works like the UseUnderline property in Gtk. I added a new sample called Mnemonics so that you can see how it works.