PR: Preserve order when saving/loading files from a project.

[rlaverde]

Fixes: #4839

When saving loading files from project configuration the order wasn't preserved, set doesn't preserve the order (except in python3.6).

Searching I found that using collections.OrderedDict is maybe the best way to delete duplicates and preserve order regardless the python interpreter implementation.

[andfoy]

I think you can use frozenset also: https://docs.python.org/2/library/stdtypes.html#frozenset

[rlaverde]

I think you can use frozenset

But frozenset doesn't guaranties order neither

[ccordoba12]

Thanks @rlaverde and sorry for the delay!