Show state and notifications on tab icon

Tab icon priority (from highest to lowest):

1. Latest Notification
   - Inactive tab: Latest notification from any view in a tab. Removed
     when tab is activated.
   - Active tab: Latest notification from focused view. Removed when
     focus changes or when the Session clears its notifications
2. Copy input or read-only indicator when all views in the tab have
   the status
3. Active view icon