This patch primarily fixes the issue that when picking the "Updateable Only" option on NewStuff.Page, we would be shown all the entries anyway. It also fixes duplicated entries on the "Installed Only" option, and further adds a little placeholder for when the view is expectedly empty.
- Use the right connections for update entry display
- While potentially expensive, only add entries to the model once
- Use Kirigami.PlaceholderMessage to show we have no entries when we don't
BUG:416762