The Client instance is now always the first argument

- xesam:artist is always a list of strings rather than a single string
- Moved the parsing logic into MusicTrackInfo

Reinstall python packages inside the virtualenv to have more control on versions

So plugins can access the notebook more easily

This clutters the logs and is rarely useful

This is useful as the PluginInstaller plugin does not need to manually
add/remove plugins inside the PluginWindow anymore

- Fix indentation

FileTransferProgress

- Make it more generic, so it fits downloads and uploads

FileTransfer

- Add DECRYPTING state
- Move state descriptions into FileTransfer
- Add cancel() method
- Add filename property

Fixes #9574, #8980

For convenience

This makes it easier for plugins to detect the nickname

Previously we called the disconnect handler only when a plugin was deactivated.

We have to preserve it until the file transfer is finished

- nbxmpp supports no HTTP Upload
- Remove support for previous HTTP Upload versions

- Refactor File class to a more generic FileTransfer class

- FileTransfer class holds the transfer state and notifies other objects via signals

- Refactor File object
- Make some methods/vars private

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