Make OptionButton::selected read-only

[raulsntos]

This PR makes the selected property read- only, the select method can be used to set the value instead.

The select method is already exposed and does the same thing as the property's setter _select_int, which is now unused so it was removed.

• Contributes to Ignoring class methods with underscore prefix cause issue with some properties #64429.

Alternative

Using the select method as the setter for the selected property is technically possible, since both methods have the same signature, and would prevent breaking compatibility. Unfortunately, this means the select method becomes hidden and I'm not sure that's what we want.

[KoBeWi]

This makes it impossible to change the property in the inspector or load it from scene. I don't think hiding select() is a big deal if the method will still be usable (so it won't break any code). Setting property does pretty much the same anyway.

[raulsntos]

I'm fine with that, but keep in mind it will also remove the documentation for the select() method since it disappears from the class reference. I can also expose a new set_selected() method for the setter that just calls select() in its implementation. But I don't have a strong preference, so whatever the team prefers.

[KoBeWi]

it will also remove the documentation for the select() method since it disappears from the class reference.

You can copy it to selected description.