make bluescreen demo more general for paging

c_os/user/demo/PagingDemo.h

@@ -0,0 +1,30 @@
+#ifndef BlueScreenDemo_include__
+#define BlueScreenDemo_include__
+
+#include "kernel/Globals.h"
+#include "kernel/threads/Thread.h"
+#include "user/event/KeyEventListener.h"
+
+class PagingDemo: public Thread {
+private:
+    void writeprotect_page();
+    void free_page();
+    void notpresent_page();
+
+    KeyEventListener listener;
+
+public:
+    PagingDemo(const PagingDemo& copy) = delete;
+
+    PagingDemo(): Thread("PagingDemo"), listener(tid) {
+        kevman.subscribe(listener);
+    }
+
+    ~PagingDemo() override {
+        kevman.unsubscribe(listener);
+    }
+
+    void run() override;
+};
+
+#endif