LibGUI: Make GUI::Application a Core::Object

Having this on the stack makes whole-program teardown iffy. Turning it
into a Core::Object allows anyone who needs it to extends its lifetime.

MenuApplets/ResourceGraph/main.cpp

@@ -159,7 +159,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    GUI::Application app(argc, argv);
+    auto app = GUI::Application::construct(argc, argv);
 
     if (pledge("stdio shared_buffer accept proc exec rpath", nullptr) < 0) {
         perror("pledge");
@@ -235,5 +235,5 @@ int main(int argc, char** argv)
 
     unveil(nullptr, nullptr);
 
-    return app.exec();
+    return app->exec();
 }