optimize OnResourceDuplicate

Optimation according Bob Ray's advice: http://forums.modx.com/thread/87517/modx--getchildids-not-working-in-onresourceduplicate-plugin#dis-post-482323. But be aware, it only works with the fixed $babel->initBabelTvById method https://github.com/mikrobi/babel/commit/7cafc9392a0df632d16a8c325af61da8f7b79299#diff-110135e006f328442d98c24b8b35b106.