revision of @polygon and @rectangle

[Knu]

Currently we have an object called roundedRect which mix up the corner type and the shape type. I am proposing* to separate the shape from the corner styling. Else we would end up with a never ending number of object for each type of corner: it doesn't scale on the long run.

For the user it doesn't change much because the shorthands (for default properties and default objects) are still available.

It also enables us to add different type of corners on @polygon.

//previously

shape: @{ 5 }

//which was equivalent to

shape: @roundedRect { corners: 5; }

//now

shape: @{ 5 }

//which is equivalent to

shape: @rectangle { corners: @{ 5 } }

//and

shape: @rectangle { corners: @round { corners: 5; }; }

*https://github.com/axr/specification/issues/83