switcherPopup: Fix scrollable check

When commit c6cea277 replaced Shell.GenericContainer, the check whether the required width exceeds the avilable width was changed from using the minimum widths of items to the natural width of the scroll view.

That doesn't work correctly, as the natural width may well exceed the actually used width: Namely the width applied by the style can restrict labels to allocate a smaller width than they would request unellipsized.

Fix this by using the minimum width of the scroll view's child instead.

Fixes #1834 (closed)