Import the RangeTree library into extern

RangeTree is a simple C++ tree set for storing non-overlapping scalar
ranges. Original source from:
https://github.com/nicholasbishop/RangeTree

Also update the build systems to include RangeTree.

source/creator/CMakeLists.txt

@@ -914,6 +914,7 @@ endif()
 		cycles_subd
 		bf_intern_raskter
 		bf_intern_opencolorio
+		extern_rangetree
 	)
 
 	if(WITH_COMPOSITOR)