Add XMLUtils.cpp and XMLUtils.h

These XML functions will be useful for de-duplicating copy-pasted XML
handling code, while also making it so elements will get updated in
place instead of being thrown out and starting from scratch every time a
file is saved.

desktop_version/CMakeLists.txt

@@ -105,6 +105,7 @@ SET(VVV_SRC
 	src/Tower.cpp
 	src/UtilityClass.cpp
 	src/WarpClass.cpp
+	src/XMLUtils.cpp
 	src/main.cpp
 	src/Network.c
 )