fix: fix parent_changed signal, and rename to native_parent_changed

[tlambert03]

This fixes #627 by properly including the widget.native.parent() as the first argument of the signal.

However, that signal name is rather confusing, since Widget.parent now mean a magicgui.widgets.Container (see #582 and #456). So, this PR also renames it as native_parent_changed, to make clearer that what the callback will receive is the native object and not a magicgui container parent. (parent_changed is retained as a deprecated alias).

This means that after widget.native_parent_changed has been emitted with a non-null value, it is still quite possible that widget.parent will be None if it is not inside of a magicgui Container. However, widget.native.parent() will not be None.

cc @aeisenbarth. hope this will help you. let me know if you have any thoughts

[aeisenbarth]

Looks good to me, thank you!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bdda89f) 87.71% compared to head (6e1202c) 87.69%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #628      +/-   ##
==========================================
- Coverage   87.71%   87.69%   -0.02%     
==========================================
  Files          40       40              
  Lines        4721     4723       +2     
==========================================
+ Hits         4141     4142       +1     
- Misses        580      581       +1     

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