Improve the source lookup engine #121

Current search engine has some issues:

1. Search Engine will resolve classes that is not in current project among workspace which makes the source mapping not compatible with project settings. (Should avoid to use SearchEngine in jdt which is not compatible with project settings. java-debug#31)
2. Search Engine cannot handle the class name with $ very well. (Should support class/package name with special characters. #19)
3. The Search policy is a kind of range search, and the result is not accurate. It could get multiple similar results for the search key so that it shows the wrong source for the stack trace. (The debugger jump to the wrong source code for Step.java #101)