feat: internal profile names (#6088)

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 :)

deltachat-ffi/deltachat.h

@@ -522,6 +522,8 @@ char*           dc_get_blobdir               (const dc_context_t* context);
  *                    In contrast to `dc_set_chat_mute_duration()`,
  *                    fresh message and badge counters are not changed by this setting,
  *                    but should be tuned down where appropriate.
+ * - `private_tag`  = Optional tag as "Work", "Family".
+ *                    Meant to help profile owner to differ between profiles with similar names.
  * - `ui.*`         = All keys prefixed by `ui.` can be used by the user-interfaces for system-specific purposes.
  *                    The prefix should be followed by the system and maybe subsystem,
  *                    e.g. `ui.desktop.foo`, `ui.desktop.linux.bar`, `ui.android.foo`, `ui.dc40.bar`, `ui.bot.simplebot.baz`.