Fix reuse of magic factory

[tlambert03]

magic_factory had a bug in which widget-specific parameters where getting cleared after the fist use of the factory 😬

this fixes that and adds a test.

[imagesc-bot]

This pull request has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/visualizing-feature-measurements-in-napari-using-colormaps-as-luts/51567/24

[codecov]

Codecov Report

Merging #224 (25b3e33) into master (279bb3b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #224   +/-   ##
=======================================
  Coverage   90.48%   90.49%           
=======================================
  Files          28       28           
  Lines        3038     3039    +1     
=======================================
+ Hits         2749     2750    +1     
  Misses        289      289           

Impacted Files	Coverage Δ
magicgui/_magicgui.py	96.29% <100.00%> (+0.06%)	⬆️

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