Move widget.{get,set}_active to AxisWidget.

The method was previously defined only for RectangleSelectors (#3375).

Also, an redundant line was removed (RectangleSelectors are already set
active on __init__ by the superclass).
As a good of an idea is it seems, because we aren't using super _every where_ can you hard-code the over loading? 

The reason I am worried is that if this ends up in an MRO where not everything is cooperating, you can get strange bugs.  If we are going to go down this route, everything in the `AxesWidget` class strucutre also needs to use `super`.