XBoard Accessibility

Dear friends, I just made the popular chess graphical front-end named XBoard accessible. We need an immediate testing. Currently it uses pico tts to announce various information. I just added KeyNavigation so that one can use arrow keys to select, press enter key to lock and use arrow keys and enter to finish move also one can use the direct notation using keyboard. The accessibility menu contains functions to announce various information.

One can do testing by cloning latest repo and switching to gtk3 branch