Allow alternate shipset mini/texture pics by splitting them up

Description

Some of the shipset pics (for instance the EmpireName_Shipset.bmp) are designed in such a way that you cannot add alternates for custom ship hulls. For instance, if I wanted to add a "gunboat" size which is between a frigate and a destroyer, I cannot specify a default icon for use in the ship lists if the icon I want is not present; I have only a single numeric index into the EmpireName_Shipset.bmp, so I am pretty much forced to use one of the stock icons such as frigate or destroyer. So I want VehicleSizes.txt to be set up so that instead of referencing a single numeric index in one big bitmap, it references three individual small bitmaps, call them EmpireName_Icon_Gunboat.bmp for the preferred icon and EmpireName_Icon_Frigate.bmp, just like all the other bitmaps except the list icons and "texture icons", whatever those are - there are three individual bitmaps for the slot layouts and the portraits, so why not for the smaller icons?