fix DefaultCameraExposureWidget widgets layout

[fdrgsp]

minor fix of how the DefaultCameraExposureWidget (and the channel widget) are displayed. Setting the SizePolicy fixed this issue

[codecov]

Codecov Report

Merging #166 (abd353d) into main (318724a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #166   +/-   ##
=======================================
  Coverage   80.66%   80.67%           
=======================================
  Files          24       24           
  Lines        2100     2101    +1     
=======================================
+ Hits         1694     1695    +1     
  Misses        406      406           

Impacted Files	Coverage Δ
micromanager_gui/_gui_objects/_tab_widget.py	93.67% <100.00%> (+0.08%)	⬆️

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