resolve ForwardRef on widget.annotation

[tlambert03]

closes #65
I think this is probably the right thing to do... We will have already tried to resolve any forward ref annotations when creating widgets, so it should already be in sys.modules and we might as well store the resolved type on widget.annotation for the convenience of others.

[codecov]

Codecov Report

Merging #66 (4732ec3) into master (510d5ef) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   77.28%   77.39%   +0.10%     
==========================================
  Files          16       16              
  Lines        1955     1964       +9     
==========================================
+ Hits         1511     1520       +9     
  Misses        444      444              

Impacted Files	Coverage Δ
magicgui/widgets/_bases.py	76.87% <100.00%> (+0.45%)	⬆️

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[sofroniewn]

Just verified this works as expected for my napari usecases in napari/napari#1856