Fix deselecting slider adding control points

[kongehund]

closes #27055

Will no longer add control points if the selection has 2 or more objects.

[bdach]

as per comment in issue thread, i'm not sure i agree with this solution.

[bdach]

Seems fine to try this and see what the response it is I guess. Low effort fix so I'm fine with reviewing.

I did add test coverage though. If you should wish to continue PRing changes to editor I'd ask to include test coverage for all functionality whenever feasible.

[kongehund]

I'd ask to include test coverage for all functionality whenever feasible.

Sure thing - I figured a one-line PR didn't warrant tests - I'll make sure to include tests in the future regardless of PR size from now.

[bdach]

When it comes to the editor we've come to the conclusion over the years that PRs dealing with selection specifics and input handling specifically need test coverage always because it is way too easy to break an existing flow with another "small one-liner change".