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/WSScreen.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <Widgets/Rect.h>
+#include <Widgets/Size.h>
+#include <Kernel/Keyboard.h>
+
+class WSScreen : public KeyboardClient {
+public:
+    virtual ~WSScreen();
+
+    int width() const { return m_width; }
+    int height() const { return m_height; }
+
+    static WSScreen& the();
+
+    Size size() const { return { width(), height() }; }
+    Rect rect() const { return { 0, 0, width(), height() }; }
+
+    static void initialize();
+
+    Point cursor_location() const { return m_cursor_location; }
+    bool left_mouse_button_pressed() const { return m_left_mouse_button_pressed; }
+    bool right_mouse_button_pressed() const { return m_right_mouse_button_pressed; }
+
+    void on_receive_mouse_data(int dx, int dy, bool left_button, bool right_button);
+
+protected:
+    WSScreen(unsigned width, unsigned height);
+
+private:
+    // ^KeyboardClient
+    virtual void on_key_pressed(Keyboard::Key) final;
+
+    int m_width { 0 };
+    int m_height { 0 };
+
+    Point m_cursor_location;
+    bool m_left_mouse_button_pressed { false };
+    bool m_right_mouse_button_pressed { false };
+};
+