Introduce LibCore and move GElapsedTimer => CElapsedTimer.

I need a layer somewhere between AK (usable both by userspace and kernel)
and LibGUI (usable by userspace except WindowServer.) So here's LibCore.

LibCore/CElapsedTimer.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <time.h>
+
+class CElapsedTimer {
+public:
+    CElapsedTimer() { }
+
+    bool is_valid() const { return m_valid; }
+    void start();
+    int elapsed() const;
+
+private:
+    bool m_valid { false };
+    struct timeval m_start_time { 0, 0 };
+};