LibCore+Ladybird: Add EventLoopManager interface for persistent state

Things such as timers and notifiers aren't specific to one instance of
Core::EventLoop, so let's not tie them down to EventLoopImplementation.

Instead, move those APIs + signals & a few other things to a new
EventLoopManager interface. EventLoopManager also knows how to create a
new EventLoopImplementation object.

Ladybird/WebContent/main.cpp

@@ -40,7 +40,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 {
     QGuiApplication app(arguments.argc, arguments.argv);
 
-    Core::EventLoop::make_implementation = Ladybird::EventLoopImplementationQt::create;
+    Core::EventLoopManager::install(*new Ladybird::EventLoopManagerQt);
     Core::EventLoop event_loop;
 
     platform_init();