Tear out or duplicate what's unique for WindowServer from Widgets.

This turned into a huge refactoring that somehow also includes
making locks recursive/reentrant.

WindowServer/WSEventLoop.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include "WSEvent.h"
+#include <AK/Lock.h>
+#include <AK/OwnPtr.h>
+#include <AK/Vector.h>
+
+class WSEventReceiver;
+class Process;
+
+class WSEventLoop {
+public:
+    WSEventLoop();
+    ~WSEventLoop();
+
+    int exec();
+
+    void post_event(WSEventReceiver* receiver, OwnPtr<WSEvent>&&);
+
+    static WSEventLoop& the();
+
+    static void initialize();
+
+    bool running() const { return m_running; }
+    Process& server_process() { return *m_server_process; }
+
+private:
+    void waitForEvent();
+
+    SpinLock m_lock;
+
+    struct QueuedEvent {
+        WSEventReceiver* receiver { nullptr };
+        OwnPtr<WSEvent> event;
+    };
+    Vector<QueuedEvent> m_queued_events;
+
+    Process* m_server_process { nullptr };
+    bool m_running { false };
+};