[Bug] macOS Big Sur + Qt 5.15.x: images are not rendered on WebEngine

[dvorka]

Bug description:

Images are not rendered on macOS w/ Qt 5.15.x - most probably due to security. Images are shown without problem on macOS w/ Qt 5.9.x

Expected behaviour:

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Environment:

• MindForger version: ...
• Operating system: ...
• Git SHA (in case of nightly/custom build): ...

Steps to reproduce :lady_beetle::

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See :lady_bug:

Screenshots:

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Error message or log:

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