Make show_file_dialog function public

[tlambert03]

just makes show_file_dialog public in magicgui.widgets

[codecov]

Codecov Report

Merging #274 (917b1f7) into master (6bc6f52) will decrease coverage by 0.30%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
- Coverage   90.26%   89.96%   -0.31%     
==========================================
  Files          28       29       +1     
  Lines        3185     3248      +63     
==========================================
+ Hits         2875     2922      +47     
- Misses        310      326      +16     

Impacted Files	Coverage Δ
magicgui/widgets/__init__.py	100.00% <ø> (ø)
magicgui/widgets/_dialogs.py	42.85% <42.85%> (ø)
magicgui/backends/_qtpy/widgets.py	85.49% <0.00%> (-0.62%)	⬇️
magicgui/widgets/_bases/categorical_widget.py	84.50% <0.00%> (-0.57%)	⬇️
magicgui/widgets/_function_gui.py	96.87% <0.00%> (ø)
magicgui/backends/_qtpy/__init__.py	100.00% <0.00%> (ø)
magicgui/type_map.py	91.42% <0.00%> (+0.09%)	⬆️
magicgui/widgets/_concrete.py	84.27% <0.00%> (+0.09%)	⬆️

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