[ENH] UserInteraction can be extended by shift.

[krystofair]

Hello,

Like in description: Added flag for keep state across whole user interactions (in UserInteraction class) and implement it for ArrowAnnotation.

I Pull Request this feature, because I think this is some kind of intuitive and decrease mouse move when we repeat the same operation.

Of course shift is my personal choice, so if ctrl be better, the change is straightforward.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.07%. Comparing base (005dc69) to head (ef53d8d).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
orangecanvas/document/schemeedit.py	66.66%	3 Missing ⚠️
orangecanvas/document/interactions.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   76.54%   76.07%   -0.48%     
==========================================
  Files         102      102              
  Lines       21316    21326      +10     
==========================================
- Hits        16317    16224      -93     
- Misses       4999     5102     +103     

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