Describe the issue

Currently the MEP22 ToolToggleBase API is that toggle-tool implementers should
implement an enable() method and a disable() method, which Matplotlib
arranges to be called when the tool is toggled. I think it would be nicer to
instead have a single on_toggle(state: bool) method.

• Having a single method avoids duplicating logic between enable() and
  disable(), given that both are often quite similar (see ToolFullScreen,
  AxisScaleBase, or GroupHideTool from the toolmanager_sgskip example).

• Using a non-verb method name makes it clearer that this is not something that
  end users can call themselves and expect to work (indeed it would cause
  problems, given that the widget state wouldn't get updated). Compare e.g.
  with Qt's approach, which is also based on implementing special method names
  in subclasses (yes, Qt also lets you bind arbitrary callables (slots) but
  that's not the MEP22 design): these overridable methods are typically named
  fooEvent() (mouseMouveEvent()), rather than do_foo(). I don't have a
  very strong preference between e.g. on_toggle(state: bool) and
  toggle_event(state: bool), although the latter may be confused with string
  event names?

This can relatively easily be implemented with a transition period using
_deprecate_method_override, I think.