Past notifications persist on disconnect then connect to different core

Test case

Steps

1. Connect to a Quassel core
2. Receive a notification
3. Disconnect from Quassel core
4. Connect to a different Quassel core that doesn't have the same buffers

Expected

Notifications that don't exist on the second Quassel core disappear.

Actual

Notifications remain while connected to the second Quassel core.

Additional

Alternatively, this could be fixed by removing notifications when manually disconnecting (not timeout/etc) from a Quassel core. Currently, notifications must be swiped away to clear up the system tray.

This will likely change once the goal of push notifications that arrive regardless of being actually "connected" exists in Quassel.