Stirling side-art: add missing radio mast
Add side-art for Supermarine Southampton (xbomber)
Add side-art for Armstrong-Whitworth Elswick (xbomber)

Based on Whitley but with the engines changed, the wing incidence reduced
 and the nose raised.
Added side-art for xbombers Selkirk and Windsor

The Selkirk artwork doesn't have many changes from the Stirling it's based on;
 ideally more should be done to make it distinctive.

The line-art on which the Windsor side-art was based has an unknown copyright/
 licensing status; the site (aviastar.org) gives no provenance or copyright
 info.  The finished side-art, however, is (as usual) a synthesis based on
 several photographs in addition to the line-art.