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).
Update background on widget.set_active(True).

Because selectors must update their background on set_active(True),
widget.active was made a property for backwards compatibility.
Updated whats_new.rst.

Because _Selector is a private class there are no references to the
actual getter and setter in the rst file.
Minor fixes according to Github discussion.
_SelectorWidget overrides the active property.

... so it should dynamically retrieve the set_active method.
Merge pull request #3376 from anntzer/widget-active-setter-getter

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