ColumnView: Clip children to prevent out-of-bounds interactions

There are cases where ColumnView does not take up all available horizontal space because there is some graphical item used for padding, such as vertical separator in System Settings. In those cases non-interactive items, even if they visually render on top, would let mouse interactions pass through them, leading to superfluous page switching and confusion.

See also: plasma/systemsettings!251 (closed)