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This is way more logical than an Action suffit for class names

This is meant to avoid opposite actions to be stacked at the same time
(for example a pause followed by an unpause, which would be a big waste of
ressources)

This probably cause regressions for now, but will be corrected soon.

This is way much cleaner, much more evolutive, and allows more advanced
operation in the future.

Doesn't undo much because of unimplemented method in the timeline GUI

This factory will enable us to create the pages for the projectWizard,
without ugly code copy/pasta