Use default QFlags construction instead of zero

[YakoYakoYokuYoku]

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PR Description

What type of PR is this? (Check one of the boxes below)

• [x] 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)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 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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