Persist parameter values across sessions

[tlambert03]

closes #152.

This allows a user to save/restore the state of a FunctionGui by using the persist=True parameter either in FunctionGui or in the @magicgui decorator. Parameter values are stored to disk (in ~/.config/magicgui/cache) when they are changed, and reloaded upon future initialization (provided persist is still True the next time it is initialized):

@magicgui(persist=True)
def func(x, y, z):
    ...

@HagaiHargil, give this a spin when you have a chance and let me know if anything needs to be changed.

[codecov]

Codecov Report

Merging #160 (7b445ed) into master (5b794f2) will decrease coverage by 0.34%.
The diff coverage is 79.01%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
- Coverage   90.85%   90.51%   -0.35%     
==========================================
  Files          28       29       +1     
  Lines        2723     2804      +81     
==========================================
+ Hits         2474     2538      +64     
- Misses        249      266      +17     

Impacted Files	Coverage Δ
magicgui/_magicgui.py	96.22% <ø> (ø)
magicgui/_util.py	63.63% <63.63%> (ø)
magicgui/widgets/_bases/container_widget.py	94.57% <94.73%> (+0.02%)	⬆️
magicgui/widgets/_function_gui.py	96.95% <100.00%> (+0.37%)	⬆️

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[HagaiHargil]

I only now had a chance to use it and it works wonderfully! Thanks.

Wouldn't you prefer the cache to be in /home/user/.cache? Library like appdirs can take care of that, although pulling a dependency for this minor feature might be unwanted.

[tlambert03]

yeah, i'd prefer to avoid the additional dependency, but I can certainly use the same destination folders as appdirs. will do, thanks for trying it out