Improve fallback behavior of tqdm iterator inside of *hidden* magicgui widget

[tlambert03]

This PR allows a tqdm iterator to fallback to the standard console behavior even when inside of an @magicgui decorated function, if the widget is not visible.

[codecov]

Codecov Report

Merging #131 (0f24c6f) into master (7f51aef) will decrease coverage by 0.05%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
- Coverage   90.81%   90.75%   -0.06%     
==========================================
  Files          26       26              
  Lines        2525     2531       +6     
==========================================
+ Hits         2293     2297       +4     
- Misses        232      234       +2     

Impacted Files	Coverage Δ
magicgui/widgets/_function_gui.py	96.59% <ø> (-0.03%)	⬇️
magicgui/tqdm.py	94.52% <85.71%> (-2.45%)	⬇️

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