Add multi-Selection widget

[tlambert03]

closes #229 by implementing a multi-select variant of the CategoricalWidget (i.e. ListWidget instead of ComboBox)

@magicgui(
    pick_some={
        "choices": ["first", "second", "third", "fourth"],
        "allow_multiple": True,
    }
)
def my_widget(pick_some=["first"]):
    print("you selected", pick_some)

[codecov]

Codecov Report

Merging #265 (a0b3e64) into master (98d7ba7) will decrease coverage by 0.21%.
The diff coverage is 76.71%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
- Coverage   90.70%   90.48%   -0.22%     
==========================================
  Files          28       28              
  Lines        3109     3164      +55     
==========================================
+ Hits         2820     2863      +43     
- Misses        289      301      +12     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/__init__.py	100.00% <ø> (ø)
magicgui/widgets/__init__.py	100.00% <ø> (ø)
magicgui/backends/_qtpy/widgets.py	86.96% <74.19%> (-0.81%)	⬇️
magicgui/widgets/_bases/categorical_widget.py	84.50% <83.33%> (-0.57%)	⬇️
magicgui/type_map.py	91.42% <100.00%> (+0.09%)	⬆️
magicgui/widgets/_concrete.py	84.27% <100.00%> (+0.09%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 98d7ba7...a0b3e64. Read the comment docs.