LibCore+Userland: Implement Core::deferred_invoke

Core::deferred_invoke is a way of executing an action after previously
queued events have been processed. It removes the requirement of
having/being a Core::Object subclass in order to defer invocation
through Core::Object::deferred_invoke.

Core::Object::deferred_invoke now delegates to Core::deferred_invoke.
The version with the Object& argument is still present but will be
removed in the following commits.

This commit additionally fixes a new places where the
DeferredInvocationEvent was dispatched to the event loop directly, and
replaces them with the Core::deferred_invoke equivalent.

Userland/Libraries/LibCore/EventLoop.cpp

@@ -394,7 +394,7 @@ void EventLoop::pump(WaitMode mode)
             }
         } else if (event.type() == Event::Type::DeferredInvoke) {
             dbgln_if(DEFERRED_INVOKE_DEBUG, "DeferredInvoke: receiver = {}", *receiver);
-            static_cast<DeferredInvocationEvent&>(event).m_invokee(*receiver);
+            static_cast<DeferredInvocationEvent&>(event).m_invokee();
         } else {
             NonnullRefPtr<Object> protector(*receiver);
             receiver->dispatch_event(event);