LibGUI: Rename GEventLoop::exit() and GApplication::exit() to quit().

These functions don't exit immediately, but rather on the next iteration
of the event loop.

Since exit() is already used by the standard library, let's call it quit()
instead. That way, saying exit() means the same thing here as anywhere else.

LibGUI/GEventLoop.h

@@ -32,7 +32,7 @@ public:
     void register_notifier(Badge<GNotifier>, GNotifier&);
     void unregister_notifier(Badge<GNotifier>, GNotifier&);
 
-    void exit(int);
+    void quit(int);
 
     bool post_message_to_server(const WSAPI_ClientMessage&);
     bool wait_for_specific_event(WSAPI_ServerMessage::Type, WSAPI_ServerMessage&);