Substitute recursive mutex enum with QRecursiveMutex

[YakoYakoYokuYoku]

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PR Description

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• Bug fix (non-breaking change which fixes an issue)
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What does this pull request do?

QMutex::Recursive was deprecated in Qt 5.14 and QRecursiveMutex is recommended to be used instead.

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Built Natron and used the node graph editor.

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[devernay]

If you prefer, you can work on feature branches on the main repo an make your PRs from there

[YakoYakoYokuYoku]

I was thinking to use my fork for more commit heavy PRs, but thanks for letting me know that I can use feature branches.