Feature/new hook - Create directories through create_filesystem_structure.

[AlexLaur]

• Create a new hook for directories creations.

We did that because we have some paths which contains some constrains in our schemas. And we need to make some actions on Shotgrid before calling create_filesystem_structure().

We tried to just modify the scene_operation hook on the operation prepare_new and save_as but it doesn't work when we try to save a new file in a task where nothing has been created before and where we don't use the New File action button.

Thanks