Unfortunately something is wrong with the Qt style, it looks like the Qt style
does not respect the rect given for the sub components
Also Input is not handled yet.
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.
Currently the Qt backend still redirect everything to the GL backend,
but the goal is to use QPainter and QWindow
This also adds a "default" backend, whose goal is to select the proper
backend at compile time
