refactor: use magicgui.types.Separator

[qin-yu]

This is perhaps the first use case of magicgui.types.Separator because I created it for this : )

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 54.72%. Comparing base (b492178) to head (44a1da2).
Report is 4 commits behind head on plantsegv2.

Files with missing lines	Patch %	Lines
plantseg/viewer_napari/widgets/predictions.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           plantsegv2     #311      +/-   ##
==============================================
+ Coverage       54.71%   54.72%   +0.01%     
==============================================
  Files              59       59              
  Lines            4200     4201       +1     
==============================================
+ Hits             2298     2299       +1     
  Misses           1902     1902              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.