* Level four is compeleted as off this revision.
   Note: I notice below warning from Android log system:
   "W - system_process - WindowManager -
   Drag is in progress but there is no drag window handle."
   Probebly this warning existed before since drag-drop code hasn't
   changed by this commit.
refs #5 Level four to eight.
 * Deeper investigation for resolving recent mentioned warning:
  "Drag is in progress but there is no drag window handle."
   One of good founded implementations regards to darg-drop was:
   https://blahti.wordpress.com/2013/03/04/drag-drop-for-gridview-v4/
   which interfaces.dragdrop package is based on. But still LogCat
   shows this warnning. Since I am testing this app in Android emulator,
   maybe this warning is because of emulator is slower than a real
   device. I decided to ignore this warning for now.
 * R.java and gen folders are excluded from source repository(Git) as of this
   revision. Note: This generated files may be included in some
   commits(e.g. releases, tags)
refs #5 Level four to eight.