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

see https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s02.html

Implement chatstate in MUC, as defined in XEP-0085 §5.5

See merge request !11