Improve multi-device support

[r10s]

Multi-device support is handled rudimentary, but there is many stuff that is not synced. Not sure if everything is really needed and possible, but I think we'll collect these things here:

• [x] Messages send on DeviceA do appear on DeviceB if the chat exists on both devices (after a delay of ~30 minutes)
• [x] check how this delay can be reduced - targeted by https://github.com/deltachat/deltachat-core/milestone/2
• [X] Chats created from external do appear on DeviceA and DeviceB
• [X] MDNs should not be sent multiple times from multiple Devices
• [x] Support Autocrypt Key Transfer
• [x] Chats created on DeviceA should also be created on DeviceB automatically (via https://github.com/deltachat/deltachat-android/issues/148)

Discussions:

• Check if messages deleted on DeviceA should/can also be deleted on DeviceB
• Check if notifications shown on DeviceA should/can disappear if the message is read on DeviceB and the message on DeviceA should be marked as read
• Check if archived/sticky chats, maybe starred messages should/can be synchronized
• Check multi-device, oob-verification and verified-groups

This issue is currently not part of the Milestone 1.0.0 as this would delay the whole thing too much.

[csb0730]

See my last comment for https://github.com/deltachat/deltachat-android/issues/148

Going on here ...

[r10s]

see also https://github.com/deltachat/deltachat-android/issues/143

[testbird]

Copied @csb0730's closed comment https://github.com/deltachat/deltachat-android/issues/148#issuecomment-341862630 and pasted this here to keep the info:

to keep it simple:

• Just let the DC show every message which is new for the local client (not existent in local database)
• If "From:" header contains own configured address show it as outgoing, but show it. If chat for incoming message not exists (at the device X) I would vote for: show it in "contact requests". From there user can start chat if desired.
• And my special wish (again ;-) ): If "To:" header contains own configured address: show it a) in chat where it comes from or b) in "contact requests" when own configured address is used in "From:". From there chat can be started if desired. This will violate no convention. Even at mail server You see a new message in INBOX in this case!

I think these three points will ease the multi device support.

[testbird]

(from #164)

If it really really (and after having tested it again) can't be avoided, multi-client support might later require deltachat to have a special status message in the email chat folder on the server, e.g. for better coordination of maintenance work.

[r10s]

closing this for now, the most important things are targeted by https://github.com/deltachat/deltachat-core/pull/407 we'll see how this works out and can created smaller issues for things that can be really worked on realistically.

for discussion in general, the forum at https://support.delta.chat should be used.