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/modulemanager.h

@@ -28,6 +28,8 @@
 class ModuleManager
 {
 public:
+    void clear();
+
     void discoverModulesPath();
     void discoverModules();
     void autoLoadModules(int maxPriority);