Add a 'Unify Corner Radius' Button for StyleBoxFlat

[brenodev777]

The "Unify Corner Radius" feature would allow users to adjust the corner radius of all sides simultaneously, rather than modifying each value separately. This would improve workflow efficiency, especially when designing rounded interfaces. By enabling a toggle, users could link all corner radius values, ensuring uniformity and reducing repetitive adjustments.

[Calinou]

Thanks for the proposal! Consolidating in #10707.