Load and Save Changed

Loader and Saver change to be platform independent, and abstract.
The proposal is to be extended by a platform dependent class that
implements the openFile method.
And create an AndroidSaver and AndroidLoader
Implemented Platform improvements

Drawing, Line, Point is now platform dependent.
Android Drawing, Line and Point implements AndroidDrawable and the
responsibility to draw is now on this classes instead of in NRDrawView
Better Responsibility Delegate

Some of the operations was removed from DrawActivity
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Merge branch 'Ricardo'

Conflicts:
	src/isel/leic/poo/nrdraw/android/NRDrawActivity.java
	src/isel/leic/poo/nrdraw/android/filesystem/FileOperation.java
	src/isel/leic/poo/nrdraw/android/filesystem/Loader.java
	src/isel/leic/poo/nrdraw/android/filesystem/Saver.java
	src/isel/leic/poo/nrdraw/android/view/DrawView.java
	src/isel/leic/poo/nrdraw/model/Drawing.java