Use QApplication.instance() instead of qApp (Qt6 compatibility)

[markotoplak]

Issue

2023-09-29 01:29:44,099:INFO:orangecanvas.registry.discovery: Could not import 'orangecontrib.associate.widgets.owassociate'.
Traceback (most recent call last):
  File "/home/frankc/orange/lib/python3.11/site-packages/orangecanvas/registry/discovery.py", line 309, in iter_widget_descriptions
    module = asmodule(name)
             ^^^^^^^^^^^^^^
  File "/home/frankc/orange/lib/python3.11/site-packages/orangecanvas/registry/discovery.py", line 552, in asmodule
    return __import__(module, fromlist=[""])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/frankc/orange/lib/python3.11/site-packages/orangecontrib/associate/widgets/owassociate.py", line 9, in <module>
    from AnyQt.QtWidgets import QTableView, qApp, QGridLayout, QLabel
ImportError: cannot import name 'qApp' from 'AnyQt.QtWidgets' (/home/frankc/orange/lib/python3.11/site-packages/AnyQt/QtWidgets.py)
2023-09-29 01:29:44,100:INFO:orangecanvas.registry.discovery: Could not import 'orangecontrib.associate.widgets.owitemsets'.
Traceback (most recent call last):
  File "/home/frankc/orange/lib/python3.11/site-packages/orangecanvas/registry/discovery.py", line 309, in iter_widget_descriptions
    module = asmodule(name)
             ^^^^^^^^^^^^^^
  File "/home/frankc/orange/lib/python3.11/site-packages/orangecanvas/registry/discovery.py", line 552, in asmodule
    return __import__(module, fromlist=[""])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/frankc/orange/lib/python3.11/site-packages/orangecontrib/associate/widgets/owitemsets.py", line 9, in <module>
    from AnyQt.QtWidgets import QTreeWidget, QTreeWidgetItem, qApp

From biolab/orange3#6588

I had exact same problems with PyQt6 6.5.

Description of changes

qApp -> QApplication.instance()

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (80b4983) 52.71% compared to head (37690eb) 52.71%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   52.71%   52.71%           
=======================================
  Files           6        6           
  Lines         791      791           
=======================================
  Hits          417      417           
  Misses        374      374           

Files	Coverage Δ
orangecontrib/associate/widgets/owassociate.py	88.77% <100.00%> (ø)
orangecontrib/associate/widgets/owitemsets.py	0.00% <0.00%> (ø)

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