GWidget: Add some new child z-ordering facilities.

- child_at(Point)
- move_to_front()
- move_to_back()
- is_frontmost()
- is_backmost()

This patch also makes it possible to receive the mouse event that triggers
a context menu before the context menu is shown. I'm not sure this is the
best design for context menus but it works for now.

LibCore/CObject.h

@@ -18,6 +18,7 @@ public:
     virtual void event(CEvent&);
 
     Vector<CObject*>& children() { return m_children; }
+    const Vector<CObject*>& children() const { return m_children; }
 
     CObject* parent() { return m_parent; }
     const CObject* parent() const { return m_parent; }