Only show DownloadItemsSheet if there's more than one download item

Summary:
This was caused by using the downloadCount (which is the total number of times the item has been downloaded), rather than the length of the list of download items.

BUG:413437

Test Plan:
With patch: Shows "Install..." for all items (with other than one user download)
Without patch: Shows "Install..." only for items with more than one download item

Reviewers: #knewstuff, #frameworks, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D25165