qt: use `QWheelEvent::DefaultDeltasPerStep` instead of hardcoded `120`

I realized recently about the existence of QWheelEvent::DefaultDeltasPerStep.

Request review @chub.

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added 139 commits

• 93d7522f...be336c08 - 138 commits from branch videolan:master
• c978d50d - qt: use `QWheelEvent::DefaultDeltasPerStep` instead of hardcoded `120` in `FlickableScrollHandler`

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