Pending messages get stuck in pending state if gajim loses connection

• gajim nightly

Steps to reproduce

1. cause the connection between gajim and server to break (with VPN, select different exit node)
2. send message while connection is broken
3. restart gajim OR wait for gajim to establish a new connection to our server

What happens

Messages sent when connection was broken are permanently shown with pending message icon.

What I expect to happen

There are multiple options.

1. pending messages to be shown with some icon indicating that the message failed to send
2. pending messages to be saved as a draft
3. pending messages to be sent upon starting up
4. pending messages to simply disappear