LibGUI: Add GWidget::doubleclick_event().

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

LibGUI/GElapsedTimer.cpp

@@ -0,0 +1,25 @@
+#include <LibGUI/GElapsedTimer.h>
+
+void GElapsedTimer::start()
+{
+    gettimeofday(&m_start_time, nullptr);
+}
+
+inline void timersub(const struct timeval* a, const struct timeval* b, struct timeval* result)
+{
+    result->tv_sec = a->tv_sec - b->tv_sec;
+    result->tv_usec = a->tv_usec - b->tv_usec;
+    if (result->tv_usec < 0) {
+        --result->tv_sec;
+        result->tv_usec += 1000000;
+    }
+}
+
+int GElapsedTimer::elapsed() const
+{
+    struct timeval now;
+    gettimeofday(&now, nullptr);
+    struct timeval diff;
+    timersub(&now, &m_start_time, &diff);
+    return diff.tv_sec * 1000 + diff.tv_usec / 1000;
+}