Prevent renaming to an unkown extension from FileSystem dock.

[jmb462]

Fixes #56805

Issue description

Renaming a file in FileSystem dock to an unrecognized extension hides the file in editor.
No warning. Just a file that disapears. Can't be undone from editor FileSystem dock.

Fix proposal

Prevent doing such a thing because it is probably a mistake and it can't be undone from the editor which may be quite misleading.
Instead of renaming, show a warning explaining the situation.
Invite user to use system file explorer to do such a rename on purpose.

As english is not my native language, you are invited to provide me a better message if you think it's needed.

[KoBeWi]

I noticed that EditorFileSystem has a list of valid extensions. Maybe we could add a method to retrieve it, so it can be reused?

[jmb462]

I noticed that EditorFileSystem has a list of valid extensions. Maybe we could add a method to retrieve it, so it can be reused?

I've added a method : Set<String> EditorFileSystem::get_valid_extensions() const
It's just returning valid_extensions. As the list is updated in scan(), I'm not sure if we need to check if valid_extension is not empty and if empty, call _update_extensions().

The function is not exposed to GDScript. Don't know if it will be very usefull (plugins ?)

[KoBeWi]

Looks good. The message might need tweaking though, CC @Calinou

As the list is updated in scan(), I'm not sure if we need to check if valid_extension is not empty and if empty, call _update_extensions().

AFAIK editor automatically scans on startup, so this is not a problem.

The function is not exposed to GDScript. Don't know if it will be very usefull (plugins ?)

No need to expose it for now.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.5.

[akien-mga]

Cherry-picked for 3.4.3.