Make vertical layout the default

[tlambert03]

closes #121 by making layout='vertical' the default for magicgui and Containers

[codecov]

Codecov Report

Merging #124 (9ab4f80) into master (b81fbf4) will increase coverage by 0.20%.
The diff coverage is 82.35%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   90.33%   90.54%   +0.20%     
==========================================
  Files          25       25              
  Lines        2443     2443              
==========================================
+ Hits         2207     2212       +5     
+ Misses        236      231       -5     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	85.45% <0.00%> (+0.51%)	⬆️
magicgui/widgets/_function_gui.py	96.35% <ø> (ø)
magicgui/widgets/_bases/container_widget.py	93.78% <100.00%> (+0.66%)	⬆️
magicgui/widgets/_bases/widget.py	87.50% <100.00%> (+0.59%)	⬆️
magicgui/widgets/_concrete.py	82.62% <100.00%> (+0.31%)	⬆️

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