Notification of ongoing call

[markcs]

After a call has been established, there should be a notification in the status bar that a call has been established and is ongoing. The notification should be removed when the call is finished.

I've had instances where I thought I'd finished the call, but NextCloud talk was still active in the background without me realizing it.

[jancborchardt]

This is quite important to give proper feedback about the state of the call. Cc @mario

[doits]

👍

This could also help to keep the call connected, even when it is in background for a longer time. E.g. connect to a call and lock the phone --> it should stay connected to the call. Right now it can get killed in background (afaik), at least this happened to me 1-2 times. A permanent notification might prevent it being killed, at least I think this is how some other apps do it.