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/CEvent.h

@@ -31,8 +31,13 @@ public:
 
     unsigned type() const { return m_type; }
 
+    bool is_accepted() const { return m_accepted; }
+    void accept() { m_accepted = true; }
+    void ignore() { m_accepted = false; }
+
 private:
     unsigned m_type { Type::Invalid };
+    bool m_accepted { true };
 };
 
 class CDeferredInvocationEvent : public CEvent {