Clutter advertises itself on X11 as implementing the _NET_WM_PING protocol,
which is needed to be able to detect frozen applications; this allows us to
stop the destruction of the stage by blocking the CLUTTER_DELETE event and
wait for user feedback without the Window Manager thinking that the app has
gone unresponsive.

In order to implement the _NET_WM_PING protocol properly, though, we need
to add the _NET_WM_PID property on the Stage window, since the EWMH states:

  [_NET_WM_PID] MAY be used by the Window Manager to kill windows which
  do not respond to the _NET_WM_PING protocol.

Meaning that an unresponsive Clutter application might not be killable by
the window manager.

Fixes bug:

  http://bugzilla.openedhand.com/show_bug.cgi?id=1748



Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>