Java Gui Builder - Windows element removed

After discussion, it was decided that only one window per XML file was more realistic for a typical use-case than multiple windows.

Effects that this decision have on the system are that the windows element is not the root of the tree anymore, window is. Second, you must now instantiate all of your general objects in the window instead of outside.

This change might break some of your existing XML files. We apologize for this, but it was a necessary evil.