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/DeferredInvocationContext.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2018-2020, sin-ack <sin-ack@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibCore/Object.h>
+
+namespace Core {
+
+class DeferredInvocationContext final : public Core::Object {
+    C_OBJECT(DeferredInvocationContext)
+private:
+    DeferredInvocationContext() { }
+};
+
+}