make vertical layout default

[tlambert03]

@jni has proposed that we make layout='vertical' be default for magicgui widgets. Curious if anyone has any objections to that? (you can, of course, still use layout='horizontal'.

@HagaiHargil, @haesleinhuepf, @sofroniewn

[sofroniewn]

Fine with me! Makes for nicer symmetry with napari default :-)

[sofroniewn]

Thinking about #111 a little more, I might also propose that we also add a stretch by default to the bottom of the widget. I personally would find that more visually appealing (we could think about making it a kwarg if people wanted control, but that wouldn't be needed for me). Curious how others feel and if they have a similar personal preference or not

[HagaiHargil]

I have no opinion on the stretch thing, but I always use layout='form' which is essentially vertical, so I agree that it's a better default.