[FIX] addons: Use QTimer.singleShot to exit/restart

[ales-erjavec]

Issue

Fix #325 (or at least attempt workaround)

Changes

Queue the exit call instead of calling it directly. This seems to fix the issue, but I am not sure why.

The problem is that the whole sequence of dialogs and installation happens before the application even enters the main event loop (QApplication.exec). Only QDialog.exec are on the stack. To properly fix this we should move the loading of workflows that are present on the command line to after the application enters the main loop.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.46%. Comparing base (e9297c5) to head (2998105).

Files with missing lines	Patch %	Lines
orangecanvas/application/addons.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
- Coverage   76.47%   76.46%   -0.01%     
==========================================
  Files         102      102              
  Lines       21248    21248              
==========================================
- Hits        16249    16247       -2     
- Misses       4999     5001       +2     

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