fix for issue #13

I don't believe there's room for confusion here. Functions in the global scope are so rare in our tech that I'd hardly assume desktop() to be anything else than a member of the superclass.
I didn't make it an issue in the pull request because in the end it doesn't matter much, but since it came up I agree with agateau here that this-> is unnecessary and fairly unusual to see in our style of OO C++ code.