obvious constraints

[Matthias Clasen]

I wonder why emeus isn't adding the 'obvious'

width >=0
height >= 0

constraints for all widgets automatically.
When I tried that, it did seem to make things a bit more stable - I see fewer assertions on negative allocations with it.

[Emmanuele Bassi]

There's no actual reason, I think it would be good to add them.

[Matthias Clasen]

#37