Move return_annotation from container to FunctionGui

[tlambert03]

It never really made sense for a Container Widget to have a return annotation. (it was only there for FunctionGui to use). This moves it over to FunctionGui

[codecov]

Codecov Report

Merging #143 (e53babc) into master (aaa4eb9) will decrease coverage by 0.01%.
The diff coverage is 96.00%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
- Coverage   91.25%   91.23%   -0.02%     
==========================================
  Files          25       25              
  Lines        2503     2511       +8     
==========================================
+ Hits         2284     2291       +7     
- Misses        219      220       +1     

Impacted Files	Coverage Δ
magicgui/signature.py	94.25% <85.71%> (-0.75%)	⬇️
magicgui/backends/_qtpy/widgets.py	85.78% <100.00%> (+0.03%)	⬆️
magicgui/widgets/_bases/container_widget.py	94.55% <100.00%> (-0.38%)	⬇️
magicgui/widgets/_function_gui.py	96.57% <100.00%> (+0.27%)	⬆️

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