The GWT debugger options are confusing

There are many checkboxes and they don't seem to correlate obviously to
- command-line options `java -jar compiler.jar --help`
- CompilerOptions.java methods/fields (which I personally use directly) https://github.com/google/closure-compiler/wiki/Using-the-Compiler's-Java-API
- service API https://developers.google.com/closure/compiler/docs/api-ref
- additional webservice options https://github.com/google/closure-compiler/wiki/Additional-Webservice-Options

All this is rather confusing. To make it slightly easier when trying to create a test case in the debugger, I suggest updating the CompilationParam enum to have additional properties which describe how they map to at least some of the above options, and setting a tooltip in DebuggerGwtMain.

I might do this in a spare moment if you indicate it will be accepted as a PR.