LibGUI: Add GWidget::doubleclick_event().

Now double-clicking an item in a GTableView or GItemView will activate it.

LibGUI/GElapsedTimer.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <time.h>
+
+class GElapsedTimer {
+public:
+    GElapsedTimer() { }
+
+    void start();
+    int elapsed() const;
+
+private:
+    struct timeval m_start_time { 0, 0 };
+};