[Shape] Generation by frequency of occurence

[vbarthel-fr]

Some shapes are simpler to learn that other: for example triangle seems to be simpler shapes than diamonds.

@jogrimst is suggesting to "list the different types of shapes by frequency of occurrence in daily life, and systematically teach the most common ones first".

That would be a good start to implement a smarter ShapeGenerator than the simple RandomShapeGenerator.

Bonus: the generator could take the mistakes of the current user to modify at runtime the shapes generated.