Use a wrapper around xinitrc to ease startup of crouton-specific apps.

While trying to tidy up my xkb branch, I came up with this wrapper around the xinitrc script.

The existing solution in xinit, that is `queuelaunch`, is fine for applications, but causes problems with my `setxkbmap` command, as it may be executed after the DE sets the keyboard model (if the user decided to set one manually), due to the `sleep 1` in `queuelaunch`.

This would also help with the clipboard branch, as we can easily execute a command before the X server exits...