Use default QFlags construction instead of zero

[YakoYakoYokuYoku]

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PR Description

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
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• My change requires a change to the documentation
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What does this pull request do?

The zero constructors in QFlags had been deprecated in Qt 5 and the default constructor is recommended instead.

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Have you tested your changes (if applicable)? If so, how?

Built Natron and used the node graph editor.

Futher details of this pull request

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