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
Add a link to the PR in front, please. Also missing a dot at the end of the line :)

Try to say exactly what changed, don't assume people know what "object-oriented X API' is, name the classes and the methods.