Proposal: multidevice and (partial) offline messages

[iphydf]

Based on #1238.

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solve multidevice and (partially) offline messages (and powersaving for mobile devices)

• add toxcore capabilities (ready to merge in PR Add toxcore capabilities to "online" packet sent to friends #1033)
• add globally uniqe msg ID (Hash) to text messages
• use FT for messages (instead of messages)
• message receipts with FT (get real received timestamps)
• real send timestamps within FT

all the above is already implemented in msgV2 Proof-of-concept (ISSUE #735)
(already impl. in toxcore and 2 clients) https://github.com/Zoxcore/c-toxcore

• make FT persistent and auto resuming by saving/loading to savefile

• implement saving of minPGC data (ready to merge PR conferences saving #1156)

• use 1*n text message sending for group chats with msgV2

• add message receipts for group chats API function for clients

• change 1-on-1 text chats to group chats (effectively removing 1-on-1 text chats).
  1-on-1 text chat is just a group chat with 2 persons

• new nodes can join or leave group chats (1-on-1 chat can turn into group chat)

• devise a way to securly group a persons device to a device-group

• only owner can add/remove devices

• removed devices can NOT join any group chats (how can this be achived?)

• removed devices will be kicked out of any chats (how can this be achived?)

• sync missed text messages (both sent and received) between all online device in a device-group.
  dont sync to "removed" devices (how can this be achived?)

• sync missed text messsages (only received) between any devices in a group chat

• add needed API calls for all these new things

how does this improve things:

• users can add a mobile device and a laptop/PC to a device group
  and leave the laptop/PC always on, and only havve the mobile device go online every 15 min.
  messages will be synced, yet mobile device will save power and bandwidth.

• users can have multiple devices, with different ToxIDs for security, and still have proper multidevice
  support. clients need to adopt UI to show it properly (see Riot UI and multiple devices)

• messages that get synced (or received later because of faux offline messaging) will have proper send
  and receive timestamp

• group message receipts and real sent and receive timestamps

[emdee-is]

You might want to look at https://git.macaw.me/emdee/tox_profile/wiki/MultiDeviceAnnouncementsPOC for a POC.