In KF6, OverlaySheet clips its children, so if the sheet is not large enough, the cancel button will be clipped. In addition, the property that indicates whether it is opened is called opened instead of sheetOpen in KF6. This commits makes enough changes for it to pass under both KF5 and KF6.

Type: fix

CCBUG: https://iron.lily-is.land/T85

Verify that unit tests pass under Qt5/KF5. Verify that tst_ConfirmUploadPopup.qml passes under Qt6/KF6.