Add more sizing options (min/max width/height)

[tlambert03]

Just adding a few more widget-sizing methods to the Protocols. (get/set min/max/current width/height) ... each of width has parallels in ipywidgets

[codecov]

Codecov Report

Merging #116 (5710b6a) into master (770efe6) will decrease coverage by 0.86%.
The diff coverage is 63.95%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   88.34%   87.47%   -0.87%     
==========================================
  Files          25       25              
  Lines        2369     2452      +83     
==========================================
+ Hits         2093     2145      +52     
- Misses        276      307      +31     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	82.47% <50.00%> (-2.19%)	⬇️
magicgui/widgets/_protocols.py	72.83% <67.74%> (-0.83%)	⬇️
magicgui/widgets/_bases/widget.py	86.90% <70.00%> (-3.68%)	⬇️
magicgui/widgets/_concrete.py	82.53% <100.00%> (ø)

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