Add widget_init parameter to magic_factory

[tlambert03]

This closes #155 by adding a widget_init parameter to magic_factory. It expects a callable that takes a single argument which will be called with the newly created widget instance each time a widget is created by the factory.

def init(widget):
    widget.x.changed.connect(lambda e: print("you changed x"))

@magic_factory(widget_init=init)
def factory(x: int = 1):
    pass

widget = factory()
widget.x.value=4  # prints you changed x

[codecov]

Codecov Report

Merging #159 (2ec3bcc) into master (a202022) will decrease coverage by 0.44%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   91.29%   90.85%   -0.45%     
==========================================
  Files          26       28       +2     
  Lines        2609     2723     +114     
==========================================
+ Hits         2382     2474      +92     
- Misses        227      249      +22     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	86.79% <50.00%> (+0.24%)	⬆️
magicgui/_magicgui.py	96.22% <100.00%> (+0.87%)	⬆️
magicgui/__init__.py	60.00% <0.00%> (ø)
magicgui/application.py	80.89% <0.00%> (ø)

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[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/51567/36