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/WSWindowManager.h

@@ -0,0 +1,95 @@
+#pragma once
+
+#include <Widgets/Rect.h>
+#include <Widgets/Color.h>
+#include <Widgets/Painter.h>
+#include <AK/HashTable.h>
+#include <AK/InlineLinkedList.h>
+#include <AK/WeakPtr.h>
+#include <AK/Lock.h>
+#include "WSEventReceiver.h"
+
+class WSFrameBuffer;
+class MouseEvent;
+class PaintEvent;
+class WSWindow;
+class CharacterBitmap;
+class GraphicsBitmap;
+
+class WSWindowManager : public WSEventReceiver {
+public:
+    static WSWindowManager& the();
+    void addWindow(WSWindow&);
+    void removeWindow(WSWindow&);
+
+    void notifyTitleChanged(WSWindow&);
+    void notifyRectChanged(WSWindow&, const Rect& oldRect, const Rect& newRect);
+
+    WSWindow* activeWindow() { return m_activeWindow.ptr(); }
+
+    void move_to_front(WSWindow&);
+
+    static void initialize();
+
+    void draw_cursor();
+
+    void invalidate(const WSWindow&);
+    void invalidate(const Rect&);
+    void invalidate();
+    void flush(const Rect&);
+
+private:
+    WSWindowManager();
+    virtual ~WSWindowManager() override;
+
+    void processMouseEvent(MouseEvent&);
+    void handleTitleBarMouseEvent(WSWindow&, MouseEvent&);
+
+    void setActiveWindow(WSWindow*);
+    
+    virtual void event(WSEvent&) override;
+
+    void compose();
+    void paintWindowFrame(WSWindow&);
+
+    WSFrameBuffer& m_framebuffer;
+    Rect m_screen_rect;
+
+    Color m_activeWindowBorderColor;
+    Color m_activeWindowTitleColor;
+
+    Color m_inactiveWindowBorderColor;
+    Color m_inactiveWindowTitleColor;
+
+    HashTable<WSWindow*> m_windows;
+    InlineLinkedList<WSWindow> m_windows_in_order;
+
+    WeakPtr<WSWindow> m_activeWindow;
+
+    WeakPtr<WSWindow> m_dragWindow;
+
+    Point m_dragOrigin;
+    Point m_dragWindowOrigin;
+    Rect m_lastDragRect;
+    Rect m_dragStartRect;
+    Rect m_dragEndRect;
+
+    Rect m_last_cursor_rect;
+
+    unsigned m_recompose_count { 0 };
+
+    RetainPtr<GraphicsBitmap> m_front_bitmap;
+    RetainPtr<GraphicsBitmap> m_back_bitmap;
+
+    Vector<Rect> m_invalidated_rects;
+
+    bool m_pending_compose_event { false };
+
+    RetainPtr<CharacterBitmap> m_cursor_bitmap_inner;
+    RetainPtr<CharacterBitmap> m_cursor_bitmap_outer;
+
+    OwnPtr<Painter> m_back_painter;
+    OwnPtr<Painter> m_front_painter;
+
+    mutable SpinLock m_lock;
+};