qt: handle high precision wheel events on playback speed widget

This handles high precision wheel events through WheelToVLCConverter, but it does not start supporting high precision adjustment (although the slider does that). I'm not sure if we want to have high precision adjustment on the button itself.

Request review @chub.

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added 152 commits

• 1b75a628...cf03e5ea - 149 commits from branch videolan:master
• e0059c3c - qml: make playback speed button has fixed width
• fb2f15e6 - qml: handle high precision adjustment and use `WheelToVLCConverter` in `PlaybackSpeedButton.qml`
• fbe1b1e8 - qml: handle horizontal orientation as well in `PlaybackSpeedButton` wheel handler

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merged