Add Show trait as requirement for Drawable impl

Suddenly realized conrod did not compile in my project any more due to a compiler error, where `val_to_string` expects the Show trait to be implemented, which the calling function did not explicitly require. I am unsure why this appears to be required, since the function val_to_string only expects ToString, which is required in the Drawable impl and furthermore only its generic implementation requires Show, it is not even a subtrait. In any case, this seems to fix it at least.