Mouse ownership and components

[corranwebster]

In the report for enthought/chaco#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.