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/core/configmanager.cpp

@@ -57,6 +57,11 @@ bool ConfigManager::save()
     return m_confsDoc->save(m_fileName);
 }
 
+void ConfigManager::clear()
+{
+    m_confsDoc->clear();
+}
+
 void ConfigManager::set(const std::string& key, const std::string& value)
 {
     if(key == "") {