fix: fix qt main window composition

[tlambert03]

fixes #685 by making _qwidget point to the actual MainWindow object (rather than the central widget) in the Qt MainWindow implementation

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.86%. Comparing base (bf9f663) to head (94b1a0e).

Files with missing lines	Patch %	Lines
src/magicgui/backends/_qtpy/widgets.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   88.85%   88.86%   +0.01%     
==========================================
  Files          39       39              
  Lines        4756     4761       +5     
==========================================
+ Hits         4226     4231       +5     
  Misses        530      530              

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