2D: Recalculate spline points when points are added

Bulk insertion signal pointsAdded was added in 6.9. That signal
belongs to XYSeries which is the parent class for line, scatter, and
spline series. When the point count changes, the spline needs to
calculate the control points. Therefore the spline connects its
calculateSplinePoints slot to XYSeries' signals emitted when the point
count changes.

Amends d49b448a5f316755dea087adf73a6a14a039c395

Pick-to: 6.12 6.11.3
Task-number: QTBUG-149245
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Change-Id: Iff0bd8ad8b20baceaadd249a1d1d4d77c6d12d0c
Reviewed-by: Sami Varanka <sami.varanka@qt.io>