Synchronize display name between devices, think over self-avatar, self-status

[link2xt]

We already synchronize status/footer when we see a self-sent message with a Chat-Version header. Would be nice to do the same for display name. There is no need for using a special sync message for this, same approach as with the status is enough.

[r10s]

disadvantage is that - same as for status - the synchronisation is delayed until a message is really sent.

if you set a name on deviceA, forget about that, and then send a message from deviceB, things are not synced - even worse, DeviceA will revert the changes.

also, not sure if "display name" isn't send unencrypted - which may be less often the case for status.

another things to be synced in this context is the self-avatar; self-avatar, self-displayname, self-status should be considered together.

so maybe a normal sync message is easier (and maybe we should prefer that also for the status)

nb: this issues should be targeted after #4953 which results in fewer questions

[link2xt]

We should add From field to the protected headers in any case, then can use it for synchronization.

[iequidoo]

We should add From field to the protected headers in any case, then can use it for synchronization.

It's already done for encrypted messages, but not for signed-only. #5033 fixes this

[iequidoo]

We should add From field to the protected headers in any case, then can use it for synchronization.

I tried to implement it this way, but it becomes too complex. Smth like Param::StatusTimestamp is needed then to reject outdated displaynames. Also this timestamp needs to be updated when Config::Displayname is set locally. Also this wouldn't work if system time isn't synchronised on devices. Also using multiple approaches to sync different config values would lead to more code and bugs while having almost no value -- it only saves some bytes and allows to sync some things w/o the synchronisation itself to be enabled. But the latter also can be a downside -- if it's usual synchonisation, you can (potentially) disable it and share the same email account across people in some organisation allowing them to have different displaynames. With syncing the displayname using the From header such a capability definitely requires a new config option.