Mouse ownership and components

In the report for https://github.com/enthought/chaco/issues/519 I suspect that we are seeing odd interactions because `BetterSelectingZoomTool` is both an overlay and a tool. I think care needs to be taken in situations where we have an interactor that dispatches to other interactors and also has mouse ownership - dispatch may be different in those cases (eg. it perhaps shouldn't dispatch to other interactors which may want to take ownership of the mouse) so for example a Component shouldn't try to dispatch to Tools it owns.