Add a method that returns a list of available flag ids

[robsch]

Suggestion: if I want to let a user choose a flag I'd like to get a list of possible ids. These could be read from the file system but I think for performance it would be better if they are hard coded in the Flags class.

Ideally you could also provide the names of the flags as well. In English but possibly in other languages as well if you can get them form somewhere.