Fix memory leaks

* Fix recursive reference memory leak in UIWidget
* Make Event/ScheduledEvent memory-leak safe
* Fix exit crashs by freeing graphics resources before destroying GL context
* Add many asserts to avoid any leak regression

src/framework/net/connection.cpp

@@ -22,6 +22,7 @@
 
 #include "connection.h"
 
+#include <framework/application.h>
 #include <framework/core/eventdispatcher.h>
 #include <boost/asio.hpp>
 
@@ -40,6 +41,7 @@ Connection::Connection() :
 
 Connection::~Connection()
 {
+    assert(!g_app->isTermianted());
     close();
 }