Refactored the DatabaseHelper to be a singleton.

The GuitarTrainerApplication class is created and initialize the DatabaseHelper
with context. The DatabaseHelper can be now accessed with DatabaseHelper.getInstance()
Improved lessons selection dialog.

The dialog takes now almost all available screen with the app visible in the background.
The items representing every lesson contain:
	* lesson title (short)
	* description (more extensive)
	* total time the lesson was run
Extended the Position2Note lesson.

* added the QuestionPosition2Note, with persistance support
* extended the LessonPosition2Note to use metrics
* added metrics visualization to LessonPosition2Note
* modified info.txt and legal.txt
removed unnecessary runOnUiThread() calls in **Lesson** classes.
Supporting metrics in Note2Position

* made QuestionNote2Position to be persistant
Removed dependencies to currentLesson as class var in ALesson subclasses