Revisit `options.override`

Now that every editor input has a `editorId`, I think we should revisit the `options.override` thing in untyped inputs and rather make it top level. Because setting the editor id should not be a optional thing but required in most cases. I.e. something like: `editorService.openEditor({ editorId: 'someEditor', resource: <some URI>})`. We can still allow to select to pick an editor somehow, but I feel that `override` should just move out of the options. I find this especially strange because in the `IEditorInput#matches` method we now have to probe onto `options.override`.