Add tvb-ext-xircuits official version to requirements.txt

Hello,

We have an official release for the tvb-ext-xircuits extension that we have previously tested in Lab Int (discussed in this PR).

Regarding the logos, we made sure that the jupyterlab logo does not get replaced anymore.

After discussing internally about the location on the left-toolbar, it would be great if we could keep it this way, as we feel like the extension would be too hidden in the context menu. Also, there are efforts to make this extension more generic for the Lab workflows, instead of just specific for TVB. We intend to replace the lego icon with something more specific in the next release, as well.

Let me know what you think.