Updated X11 contrib API

I've updated the jnacontrib.x11 to fix a few bugs and add in a feature.  Eclipse also reformatted the code, so there are some minor whitespace differences.

Bugs fixed:
-X.getXXXProperty() will now return null if the property is not found(see: http://tronche.com/gui/x/xlib/window-information/XGetWindowProperty.html)

New features:
-Added the ability to get all of the subwindows of an X.Window
-XDesktopDemo GUI now has a button to display all of the subwindows of the selected window in a dialog box
I could write tests; the problem is that because this is X programming, any tests are going to be highly dependent on what window manager is running and what windows are open at the time, and as far as I'm aware there's no good way to test GUI-related elements like this.