Add support for systemd's networkd

See merge request !91

https://xmpp.org/extensions/xep-0184.html#what tells us that we
SHOULD NOT impute any meaning to the fact that we did not receive an ack message

- We want to send requests also to offline contacts.
- The XEP is wide spread chances are high that a contact supports it.
- It gets really complicated, when we want to guess if a offline contact supports receipts

- 'received'-Carbons from ourself, because we either received
   the message directly or got a 'sent'-Carbon about it
-  Messages from our own full jid, this can happen when we send
   a Message to ourself and no other resource is online.

Disabled accounts are not in gajim.connections, which leads to a
key error.

Fix mediated groupchat invites

Closes #8603 and #8468

See merge request !92

- Push Invitation and Decline event much earlier
- Refactor Logic to decide if mediated or not
  Ejabbered 17.03 does add a 'jabber:x:conference' namespace into mediated invites which has broken our logic
- Dont add a groupchat contact on invitation, always add the inviter to the roster

gir1.2-gupnpigd-1.0 is the correct python3/GTK3 dependency on Debian

Port Network Watcher to Gio API

See merge request !88

O is already used for the OK button

Fix IBB

Closes #8196

See merge request !90

- Move handling of Data IQs into IBBIqHandler
- Call SendHandler with file_props
- Save last sent id in file_props.syn_id
- Remove some useless checks, now that we call SendHandler with file_props
- Send item-not-found error on invalid session id

Logs.db is always at data_root, even if we set another config directory with -c

fix "Unable to load icon gajim" errors caused by !84

See merge request !87

use 'reverse DNS' convention for desktop and icon files

See merge request !84

HistoryManager was not finding the Logs.db when Gajim was
used with the -c config path argument

fix python errors in client zeroconf

See merge request !82

Disable button if password changing is not supported

See merge request !85