Adds outside-staff-interface and outside-staff-axis-group-interface (issue 37950044)

[dak]

The summary seems incompatible with
<URL:http://music.stackexchange.com/a/14160/8773>

Once an interface is required for outside-staffing a grob, the set of
grobs one can use in that manner is hardwired to the "intended" grobs.

https://codereview.appspot.com/37950044/