vertically center dialogs

[luapmartin]

Resolves: vertically center dialogs by adding repositionWindowIfNeed method to DialogView and trigger during size update

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[Eism]

I don't think you'll be using this in qml, so let's remove Q_INVOKABLE. (For popups, it's more of a hack)

[luapmartin]

I need to adjust that PR to only center dialog "on spawn" not "on resize"