[ENH] Add 'Donate' application url role

[ales-erjavec]

Issue

Ref biolab/orange3#7142 and #331

Changes

• Add 'Donate' url role
• Display appropriately
• Also add feedback, bug report and donate to help menu (when present)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.51%. Comparing base (74729a5) to head (1bc722e).
⚠️ Report is 36 commits behind head on master.

Files with missing lines	Patch %	Lines
orangecanvas/application/canvasmain.py	76.92%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   76.48%   76.51%   +0.02%     
==========================================
  Files         102      102              
  Lines       21265    21292      +27     
==========================================
+ Hits        16265    16292      +27     
  Misses       5000     5000              

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