As a result
- The error messages will now show the error with `-` instead of `_`
- The LSP will auto-complete with -
- The interpreter's list of properties will list the property with '-'
(but we made the change so that set_property, get_property, and so on
work also if passed a '-')
The Rectangle element has properties for a border outline. If those are
used, then the generated code should use BorderRectangle. But if they are
not used, then we can fall back to just generating a Rectangle.
