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/Forward.h

@@ -15,6 +15,7 @@ class ConfigFile;
 class CustomEvent;
 class DateTime;
 class DirIterator;
+class DeferredInvocationContext;
 class ElapsedTimer;
 class Event;
 class EventLoop;