* Attribute name "active" can be confused with parameter to `RadioButtons`, but `RadioButtons` doesn't save that parameter as an attribute, so there's no name clash. BUT this is still really confusing to users. Consider renaming `AxesWidget.active`.
* * This could have been implemented as a decorator, but some callbacks wouldn't be compatible. For example, `SpanSelector.release` should not be ignored if widget was deactivated during mouse press.
* * `SpanSelector` and `RectangleSelector` already had `ignore` methods, but they do not call it in `update_background`. I don't change the current behavior, although it seems desirable.