Query screen geometry using static QGuiApplication methods

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PR Description

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

• 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?

QApplication::desktop was deprecated in Qt 5.11 and QGuiApplication::screens (or similar) is recommended to be used instead.

Show a few screenshots (if this is a visual change)

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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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