plasmoidviewer/CompactApplet: Sync expandedItem's visibility with plasma-desktop

It should've been ported from fromCurrentTheme to fromCurrentImageSet during KSvg refactoring. But at the same time, plasma-desktop counterpart doesn't have this extra condition, and with it (in a ported form) I can't get this item to show up somehow.

See also: https://invent.kde.org/plasma/plasma-desktop/-/blob/ea7398640169bb77e709820e7622011bb1751252/desktoppackage/contents/applet/CompactApplet.qml#L116

CC @niccolove @mart