Add command-line argument --no-shadow

[janezd]

Install an event listener to set NoDropShadowWindowHint to all Orange windows. I don't know whether there's a simpler way to pass options to widgets. Doing it this way also removes shadow from any windows that are not Orange widgets, which is desirable.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 35.71429% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.19%. Comparing base (9b12b1d) to head (8b2fba5).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
orangecanvas/main.py	35.71%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   76.14%   76.19%   +0.05%     
==========================================
  Files         102      102              
  Lines       21321    21342      +21     
==========================================
+ Hits        16234    16261      +27     
+ Misses       5087     5081       -6     

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