(low) Add borderStyle property

[hatton]

• Add borderStyleName property
• Add getBorderStyleNameChoices() method that returns the set of supported style names.

All we need is a set of strings like : "solid", "dashed", "dotted".

These can map to hard-coded paperjs strokes. In the future we can add a different property that allows you to have access to the details of the stroke, but this property will still allow for a cheap UI.