Implementation of new EWMH spec

[Soeren Sandmann <sandmann daimi au dk>]

Hi,

The attached patch implements all the properties dealing with
compositing managers that were recently added to the EWHM spec.

New features:

        - Override-redirect windows (POPUP windows in gtk+ terms) get
          the _NET_WM_WINDOW_TYPE property set on them

        - GdkScreen gains a signal, "composited_changed", that is
          emitted when a compositing manager comes and goes

        - GdkScreen gains a new method gdk_screen_is_composited() that
          indicates whether a compositing manager is running.

        - Widgets also gain a "composited_changed" signal. 

        - Widgets are invalidated when the compositing manager comes
          or goes away.

Notes:

        - The win32 version of gdk_screen_is_composited() just returns
          FALSE; the quartz version just returns TRUE. I don't know if
          that's even remotely correct.

        - There is a field in GtkWindow containing the
          GdkWindowTypeHint for the window. This field is only 3 bits
          wide, which is not enough for the extended enum, so instead
          I am storing the enum in GtkWindowPrivate, and maintains a
          shadow copy in GtkWindow. This shadow copy is
          GDK_WINDOW_TYPE_HINT_NORMAL if the real hint is one of the
          new types. 

I hope to get this into 2.10. Comments appreciated.

Thanks,
Soren