Refactor item export/import functionality out of Content Library

Importing and exporting items doesn't really have anything to do with Content Library, except maybe sharing some functionality for bundle handling. Currently it is necessary to ensure Content Library is visible when using import/export functionality, which is weird as nothing seems to happen in Content Library.

Relevant custom notifications: "export_item_as_bundle", "export_material_as_bundle" and "import_bundle_to_project"

This functionality should be refactored out of Content Library so it's no longer dependent on Content Library being attached.