Hook up the PS2MouseDevice to the AbstractScreen+WindowManager.

Render the mouse cursor by xor'ing the pixels. I don't know anything about
hardware cursors yet and this way we don't need to recompose the window
hierarchy every time you move the mouse. :^)

Kernel/PS2MouseDevice.h

@@ -3,11 +3,17 @@
 #include <VirtualFileSystem/CharacterDevice.h>
 #include "IRQHandler.h"
 
+class MouseClient;
+
 class PS2MouseDevice final : public IRQHandler, public CharacterDevice {
 public:
     PS2MouseDevice();
     virtual ~PS2MouseDevice() override;
 
+    static PS2MouseDevice& the();
+
+    void set_client(MouseClient* client) { m_client = client; }
+
 private:
     virtual bool has_data_available_for_reading() const override;
     virtual ssize_t read(byte* buffer, size_t buffer_size) override;
@@ -23,6 +29,13 @@ private:
     void wait_then_write(byte port, byte data);
     byte wait_then_read(byte port);
 
+    MouseClient* m_client { nullptr };
     byte m_data_state { 0 };
     signed_byte m_data[3];
 };
+
+class MouseClient {
+public:
+    virtual ~MouseClient();
+    virtual void did_receive_mouse_data(int dx, int dy, bool left_button, bool right_button) = 0;
+};