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.
Into the previous Rectangle and BorderRectangle that allows (rounded) borders through a stroke in addition to the fill.
Next step is to make the compiler make the choice automatically depending
on the properties used.
There's still an open question about how to avoid the border properties
for all rectangles but allocate them only for those who have a border.
Regardless, this border illustrates the issue with the lack of text sizing.
