Show that mailbox has sub-mailbox with unread messages

[ChristophWurst]

Is your feature request related to a problem? Please describe.

A mailbox with unread messages shows a counter. If I organize my mailboxes in a hierarchy, I do not notice that a child of a top level mailbox has unread messages if the top level mailbox is collapsed.

Example

• Notifications (0)
  • Nextcloud (17)
  • Server (3)

^ The Notifications mailbox would not indicate any unread messages in its children.

Describe the solution you'd like

Highlight that a mailbox has unread items.

Evolution does this

Roundcube does this

Describe alternatives you've considered

No response

Additional context

No response