fn focus_tag_change<H: Handle>(state: &mut State<H>, delta: i8) -> Option<bool> {
    let current_tag = state.focus_manager.tag(0)?;
    let tags = state.tags.normal();
    let relative_tag_id = relative_find(tags, |tag| tag.id == current_tag, i32::from(delta), true)
        .map(|tag| tag.id)?;
    state.goto_tag_handler(relative_tag_id)
}
Focus the adjacent tags, depending on the delta.
A delta of 1 means "next tag", a delta of -1 means "previous tag".
leftwm-core/src/handlers/command_handler.rs