UI: Cleanup frontend event handling

This adds a function to OBSBasic to call on_event, so every
time a event is called, the api variable doesn't have to be
checked everytime.

UI/window-basic-main-screenshot.cpp

@@ -57,9 +57,7 @@ ScreenshotObj::~ScreenshotObj()
 
 			main->lastScreenshot = path;
 
-			if (main->api)
-				main->api->on_event(
-					OBS_FRONTEND_EVENT_SCREENSHOT_TAKEN);
+			main->OnEvent(OBS_FRONTEND_EVENT_SCREENSHOT_TAKEN);
 		}
 	}
 }