[UX] Can't mark all as unread

[Ryuno-Ki]

Picking up from Mastodon I finally have the time to fill a proper issue for this.

Using Handy News Reader I'm used to find a checkbox in the top bar that allows me to easily mark a notification as read or unread. Currently there is only a way to mark all as read (and some cumbersome workaround to mark all as unread - more than what I selected).

From an UX point of view it is better to allow for easy undo.