internal profile names

[r10s]

this PR allows setting a "private tag" for a profile, see https://github.com/deltachat/deltachat-android/pull/3373 for a possible UI.

currently, the core does not do anything with the tag (so, it could also be a ui.-config option), however, this may change in the future - it might bet synced, and become also otherwise useful in core. also, having this in core is better documentation-wise, as otherwise each UI easily does its own things :)