Made Plug flags serialisation more future proof.

Our previous method meant that we couldn't introduce new default flags in the future, because they would default to off when loading old files. This new approach always serialises by adding and removing from Plug::Flags::Default to avoid this problem.

Pleasingly, this also reduces the file size of a large graph with many shaders by over 20%, and reduces the loading time by almost 10%.

Fixes #684.