install css style file

See merge request !97

New Encryption Plugin API

See merge request !96

- Log status is a feature that only works with xep-0136 which nobody uses anymore
- Even then we can only trust in the server/user to not log the session
- We should not imply that a client could control if messages are logged on the server or the other end,
 especially as this is about encryption and security we should be as accurate as possible

- The lock image is only shown if a encryption was sucessfully activated, so the enabled flag is unnecessary

- Add extension points when receiving/sending a message
- Add extension point for setting the lock image
- Add extension point after clicking the send button
- Add extension point when typing
- Add a new menu button to choose the desired encryption
- Extend the PluginManager to hold a list of encryption plugins

appdata: remove zero width space (U+200B) add by mistake

See merge request !95

add AppData file

Closes #8174

See merge request !94

fixes #8174

Use account hostname for STUN server discovery

See merge request !93

Use the account hostname instead of the target hostname of the
_xmpp-client._tcp SRV resource record for STUN server discovery.
Otherwise, discovery fails if both hostnames are different.

If the user had an empty `file_transfer_proxies` config setting
no proxies were used. As we discover the default server proxy ourself the
user should not have to write it to his config setting to make use of it.

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