We really shouldn't require specifying a color for every Text {} element just in order
to see *some* text.
For Rectangle OTOH transparent is a good default (and thus for Color), hence
this change just to Text.
Right now the constructor bit is also a bit repetitive, this could
perhaps be folded into BuiltinItem to generate the ffi, default impl and
forward to an init function if it exists.
The printer demo looks slightly less boring on white background with brighter colors.
In exchange, the controls can become more dull and hopefully easier to see.
