Improve issues with widget visibility

[tlambert03]

This PR fixes a few bugs around widget visibility, such as an initial disagreement between the widget.visible attribute and the actual visibility of the widget.
It also makes sure that widgets (and their labels) explicitly hidden with widget.hide() are not shown in containers, but can be reshown on demand.

[codecov]

Codecov Report

Merging #130 (03f2364) into master (3d1d3b0) will increase coverage by 0.02%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   90.78%   90.81%   +0.02%     
==========================================
  Files          26       26              
  Lines        2517     2525       +8     
==========================================
+ Hits         2285     2293       +8     
  Misses        232      232              

Impacted Files	Coverage Δ
magicgui/widgets/_bases/widget.py	87.28% <94.11%> (-0.22%)	⬇️
magicgui/backends/_qtpy/widgets.py	85.45% <100.00%> (+0.25%)	⬆️
magicgui/widgets/_bases/container_widget.py	93.78% <100.00%> (ø)
magicgui/widgets/_concrete.py	82.28% <100.00%> (+0.13%)	⬆️
magicgui/widgets/_function_gui.py	96.62% <100.00%> (+0.02%)	⬆️
magicgui/widgets/_protocols.py	99.45% <100.00%> (ø)

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