Describe the issue

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

• Having a single method avoids duplicating logic betweenenable() and
  disable(), given that both are often quite similar (seeToolFullScreen,
  AxisScaleBase, orGroupHideTool from thetoolmanager_sgskip example).

• Using a non-verb method name makes it clearer that this is not something that
  end users cancall 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 thando_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.