KF6: DownloadItemsSheet: Fix scrolling

OverlaySheet documentation clearly states:

It needs a single element declared inside, do not override its contentItem

…and KNewStuff DownloadItemsSheet.qml happily does just that:

Kirigami.OverlaySheet { id: component

//...

contentItem: QtLayouts.ColumnLayout {
    QtControls.ScrollView {
        // ...

Second part of the problem is that the docs suggests using either a ColumnLayout or a ListView as that single element. But not combining them one inside another, and especially not a ListView wrapped in an extra ScrollView inside a ColumnLayout. Moving extra top items into a ListView's inline header, on the other hand, works great, and fully fixes this scrolling issue.

Note: In order to backport to kf5 branch, replace icon.name: in Kirigami.BasicListItem delegate with older icon: counterpart.

BUG: 448800 FIXED-IN: 5.104