Encrypted communication is uni-directional

[iNPUTmice]

The issue and the cause is described here: https://github.com/omemo/gajim-omemo/issues/50

Steps to circumvent the problem include:

• [ ] rename add_devices to set_devicesbecause this is what the method actually does. Helps to prevent confusion.
• [ ] rename add_own_devices to set_own_devices (same reason as above)
• [ ] create a new method call add_device(jid, deviceId) (Should not duplicate device ids in the array. Ie treat the array as a set)
• [ ] In decrypt_msg call add_device with sender_jid and sid
• [ ] Somewhere in init (before we connect to a server) add all recipient_id, device_id combos from the litesessionstore to the device_ids array (basically calling set_devices)