Do not allow to import the project in itself

As in this comment, it is currently possible to embed a project into itself recursively.

• When importing: Forbid the user to import the current project
• When loading the project file, remove such recursive imports if they are already present (they can also happen e.g. when importing B.kdenlive in A.kdenlive and then saving A.kdenlive as B.kdenlive)

We probably do not have to check this over multiple levels as this is already a special case.