PLUGIN event fixes


**[PLUGIN] fix impossibility to catch EVT_STOP and EVT_EXIT in plugins …**

Basically with the new event dispatch system (func mapping) EVT_STOP and EVT_EXIT were missing func mappings (they are registered in the plugin.py module and not from plugin instances). I added 2 hooks `onStop(self, event)`, `onExit(self, event)` so it's possible to catch such events by overriding those methods (obviously it's also possible to use the old `onEvent(self, event)` method (retrocompatibility)