Unique Range Selector Steps

To fix overall widget behavior, steps that are attached to a range selector can no longer be attached to other widgets.

Previously, creating a step and assigning it to a range selector and other widgets caused the widgets to behave unpredictably. To fix that, a range selector can now only have its own step. A step that’s attached to a range selector can no longer be attached to any other widget, and steps that are attached to other widgets can’t be attached to a range selector.