[ENH] Save workflow image to SVG

[ales-erjavec]

Issue

Fixes biolab/orange3#6777

The results are less then stellar though. All SVG icons get rasterized.

Changes

• Save workflow image to SVG
• Also make the save ows dialog window modal instead of application modal.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.12698% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 75.29%. Comparing base (8b47de1) to head (db5216a).

Files	Patch %	Lines
orangecanvas/application/canvasmain.py	82.14%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
- Coverage   76.53%   75.29%   -1.24%     
==========================================
  Files          99       99              
  Lines       21043    21097      +54     
==========================================
- Hits        16105    15886     -219     
- Misses       4938     5211     +273     

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