Add ability to mark entries as read when adding a feed.

• Adds "Mark entries as read" checkbox to the "Add Feed" dialog and will mark entries as read/unread depending on whether it's checked
  • this is done only for the first add fetch (set's default arg of false to keep the old behavior if any of the functions are used elsewhere)
• Updates entries empty message to include "unread" if the unread filter is set
  • Before this change users would usually start out with some unread entries after adding a feed and then would naturally mark their entries read and slowly find the "Only Unread" filter but now that a user can start out with no unread entries after adding a feed, I thought it would help the UX to add text to the empty message as another indication that there's the "only unread" filter selected (helped me a few times).

*This is related to https://invent.kde.org/plasma-mobile/alligator/-/issues/30 but this isn't adding the ability to mark a current feed's entries as read, instead this MR adds the ability to mark the entries as read only when you add a feed. I imagine that most of the unread entries are added at this time and so this seemed like the most benefit from a small change.