Implement row::Wrapping widget

[hecrj]

If you have a Row, simply call Row::wrap at the end to turn it into a Row that will wrap its contents.

row![a, b, c, d, e, f, g].spacing(10).wrap()

The original alignment of the Row is preserved per row wrapped.

The layout example now showcases its use:

row((1..10).map(|i| square(if i % 2 == 0 { 80 } else { 160 })))
    .spacing(20)
    .align_y(Center)
    .wrap()