Profiler: Display timing information in ProfileTimelineWidget

Currently, there is no way to know when in a profile's duration a
sample was taken. This commit adds a basic timestamp to the timeline
widget, and a black bar to show where the cursor is hovering over.

Userland/DevTools/Profiler/ProfileTimelineWidget.h

@@ -50,4 +50,5 @@ private:
     bool m_selecting { false };
     u64 m_select_start_time { 0 };
     u64 m_select_end_time { 0 };
+    u64 m_hover_time { 0 };
 };