Rewrite debug output of event classes.

- Introduce functions to return class name and type name by type.
- Move QDebugStateSaver up.
- Add verbose output for DnD, tablet, native gestures,  context
  menu, enter and expose events.

Task-number: QTBUG-38858

[ChangeLog][QtGui][QEvent] The debug output for events has
been significantly improved.

Change-Id: I3f11e34c3c9d6a3c6db596be4051b801f00e6153
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>