Part of #8787.

The default language cannot be `None` if `app.is_installed('GSPELL')` is `True`,
because to be `True` at least one language needs to be available.

If `app.is_installed('GSPELL')` is `False`, the modified code is not called.

Fixes #10125

Fixes #10083

Fixes #10074

Fixes #10106

Adds a queue for executing background tasks

For example fetching avatars or disco info

`new_version` passed to `update_config()` corresponds to the currently
running version of Gajim, i.e. it's the latest version compared to the
other versions mentioned in `update_config()`, that's why the
conditions `V('...') <= new_version` are always true and can be removed.

https://www.python.org/dev/peps/pep-0440/

IPython 6.5 is the first version that is Python 3.7 compatible.
https://ipython.readthedocs.io/en/stable/whatsnew/version6.html#ipython-6-5-0

Follow https://gitlab.gnome.org/GNOME/Initiatives/-/issues/14

Thanks codespell!