LibCore+LibGUI+WindowServer: Make events bubble up through ancestors

With this patch, CEvents no longer stop at the target object, but will
bubble up the ancestor chain as long as CEvent::is_accepted() is false.

To the set accepted flag, call CEvent::accept().
To clear the accepted flag, call CEvent::ignore().

Events start out in the accepted state, so if you want them to bubble
up, you have to call ignore() on them.

Using this mechanism, we now ignore non-tabbing keydown events in
GWidget, causing them to bubble up through the widget's ancestors. :^)

Libraries/LibCore/CObject.h

@@ -48,6 +48,8 @@ public:
     template<typename T, typename Callback>
     void for_each_child_of_type(Callback callback);
 
+    bool is_ancestor_of(const CObject&) const;
+
     CObject* parent() { return m_parent; }
     const CObject* parent() const { return m_parent; }
 
@@ -71,6 +73,8 @@ public:
 
     static IntrusiveList<CObject, &CObject::m_all_objects_list_node>& all_objects();
 
+    void dispatch_event(CEvent&, CObject* stay_within = nullptr);
+
 protected:
     explicit CObject(CObject* parent = nullptr, bool is_widget = false);