Function add_key_at

fn add_key_at(
    project: &mut Project,
    target: Target,
    state: &mut CurvesState,
    out: &mut CurvesResponse,
    undo: &mut dyn FnMut(&Project),
    t: f64,
    v: f64,
)

Add (or move) a key of the active property at clip-local time t and value v, selecting it. Used by both the double-click and the right-click-on-empty-graph gestures.