Implementation notes: * Add variable in `list` function: [ContactControllerImpl/list](https://code.briarproject.org/briar/briar/-/blob/3c63fecb5dad2a3f8174ae40b804679780644a00/briar-headless/src/main/java/org/briarproject/briar/headless/contact/ContactControllerImpl.kt#L78) * Get data from [GroupCount](https://code.briarproject.org/briar/briar/-/blob/master/briar-api/src/main/java/org/briarproject/briar/api/client/MessageTracker.java#L79) Needed for https://code.briarproject.org/briar/briar-gtk/-/issues/29.