Add MenuExtra for `xvkbd` virtual keyboard

[morgant]

I occasionally find I need to enter accented characters for which I haven't modified my X11 keymaps for (see xmodmap(1)). A while ago I installed xvkvd, an X11 virtual keyboard, and will occasionally launch it when I need to enter accented characters. It would be extra convenient if I made an 'MenuExtra' for it so I could just launch it in a single click.