saving previous lesson type in shared prefrenced
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added Layer'ed support for drawing on the fret image.

Now the dots drawn from one source - like, touches on the screen - will not disturb the
 dots writte from other source - like, from a lesson.
added support for manual selection of the GridShape in ShapeDegree2Position lesson
Introduced the metrics classes for lessons and questions.

* added classes for lesson and question metrics
* modified layout of the learning status widget, moved it to custom view
* used LessonShapeDegree2Position to see how metrics measurement works
* remove a handful of classes, which were intended but never used for metrics measurement
Mainly layout changes:
* the Degree/Shape views: compacted buttons so that the views takes less overall screen
* NotesView: remove buttons, note selection only with gestures
* LearningStatusView: moved on the screen, placed here name of the current lesson
* in main view, the name of lesson selection button is left always unchanged