Add Inbox/Reminders/Notifications/Activity Feed

Add a place where we can display and store items for:

• subscription requests / unsubscribed
• group chat invitations / declines
• Gajim updates (Windows) / Gajim plugin updates
• message reminders (specific messages we want to reply to, but later)
• messages where we have been mentioned in group chats (see #11209)
• server messages (e g. upcoming maintenance or errors, see #10846)

Clicking on a row should start an applicable action, e.g. for reminders, it should jump to the bookmarked message and offer a checkmark button on the message row to dismiss the reminder.

Every item in this section should be easily dismissible, including bulk actions for similar items (e g. dismiss all subscription requests).

Items should be stored in order to be available after a restart (see #11592 and #11591 (closed))