* add basic multi-device-sync functions
* generate json
* add context.parse_sync_items()
* add context.execute_sync_items()
* piggyback sync-commands message, add body for human-readable part
* avoid double json renderings
* mimeparser parses incoming .json sync-files
* do not piggyback sync-files
* execute sync items
* return status of send_sync_msg()
* send sync messages as multipart/report
* add a per-item-timestamp and also allow adding other per-item-fields in the future
* if the self-chat does not exist, create it blocked/hidden
* create tokens closer to real qr-code needs
* respect bcc_self setting, add test for that
* sync qr code tokens after promoting groups
* send sync-messages only if an experimental switch is set
* trigger send_sync_msg() after sending messages and after creating/redraw/revive qr-code
* add DC_STR_* constants to deltachat.h
* adapt to refactored qr module as of #2729
* tweak test
* use SendSyncMsgs config name instead of SendExperimentalSyncMsgs - we can remove or rename the config nevertheless, but have the option to keep it without renaming
* tweak docs
* remove currently unused effective timestamp calculation
* clarify when send_sync_msg() is called
* make sure, sync-messages are encrypted and are sent by SELF
* tweak docs, fix typos