LibCore: Allow widgets to reparent

Author: https://github.com/Torstennator
Commit: 38974b4128
Pull-request: https://github.com/SerenityOS/serenity/pull/21511
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/cubiclove
Reviewed-by: https://github.com/kleinesfilmroellchen

Userland/Libraries/LibCore/EventReceiver.h

@@ -134,6 +134,7 @@ public:
     {
         if (m_parent)
             m_parent->remove_child(*this);
+        m_parent = nullptr;
     }
 
     template<class T, class... Args>