Recursive permissions on a folder do not work when just opening the scene file

[webbertakken]

There are three way to solve this:

1. ask permission twice (illustrated on the screenshot)
2. only ask permission (once) on the folder that contains the scenes recursively, support with a scene selector in the UI
3. asking the user to put the files in scope (as described in https://tauri.app/v1/api/js/fs#security)

Point 3 is what I've done as a first iteration.
Point 2 is probably best for a version 2 of this app, which requires a few more changes in the ui and the apps data model.