LibCore: Big first step towards pluggable Core::EventLoop

The EventLoop is now a wrapper around an EventLoopImplementation.
Our old EventLoop code has moved into EventLoopImplementationUnix and
continues to work as before.

The main difference is that all the separate thread_local variables have
been collected into a file-local ThreadData data structure.

The goal here is to allow running Core::EventLoop with a totally
different backend, such as Qt for Ladybird.

Userland/Libraries/LibCore/ThreadEventQueue.cpp

@@ -92,7 +92,7 @@ size_t ThreadEventQueue::process()
             case Event::Quit:
                 VERIFY_NOT_REACHED();
             default:
-                dbgln("ThreadEventQueue::process: Event of type {} with no receiver", event.type());
+                // Receiver disappeared, drop the event on the floor.
                 break;
             }
         } else if (event.type() == Event::Type::DeferredInvoke) {