Sort blueprint plugins after all others

Also validate that no non-blueprint masters or non-masters are expected
to load after any blueprint masters.

The addition of a new EdgeType may be considered a breaking change to
the API, since it can result in the caller receiving a value it doesn't
know how to handle, but it doesn't break ABI compatibility. LOOT has
explicit handling for unknown values, so is fine.

include/loot/enum/edge_type.h

@@ -46,6 +46,7 @@ enum struct EdgeType : unsigned int {
   recordOverlap,
   assetOverlap,
   tieBreak,
+  blueprintMaster,
 };
 }