KPropertiesDialog: Wrap size buttons in a QWidget

There's a separator at the end which you cannot hide (there's no hide() on neither QLayout, nor a separator), so the GridLayout reserves a small gap for this row, even if all the buttons are hidden.

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@ahmadsamir

Before (Note the gap between "Größe" and "Erstellt")

Can I have this in kf5, too, merging and diffing UI files is a nightmare, so I’d rather like this to be identical on both branches :)

After: