gtk: Add a way to do event capture

This patch adds a capture phase to GTK+'s event propagation
model. Events are first propagated from the toplevel (or the
grab widget, if a grab is in place) down to the target widget
 and then back up. The second phase is using the existing
::event signal, the new capture phase is using a private
API instead of a public signal for now.

This mechanism can be used in many places where we currently
have to prevent child widgets from getting events by putting
an input-only window over them. It will also be used to implement
kinetic scrolling in subsequent patches.

http://bugzilla.gnome.org/show_bug.cgi?id=641836

We automatically request more motion events in behalf of
the original widget if it listens to motion hints. So
the capturing widget doesn't need to handle such
implementation details.

We are not making event capture part of the public API for 3.4,
which is why there is no ::captured-event signal.

gtk/gtkwidget.h

@@ -437,7 +437,6 @@ struct _GtkWidgetClass
   void (*_gtk_reserved5) (void);
   void (*_gtk_reserved6) (void);
   void (*_gtk_reserved7) (void);
-  void (*_gtk_reserved8) (void);
 };
 
 struct _GtkWidgetAuxInfo