The defaults were not parsed properly when a list with a single item was given as teh default as it could not determine if it was a text or floating-point values.

- Refactor functions used to create the USD file format list.
- Provide function to retrieve different USD file format labels.
- Provide functions to get and set (load and save) the last-used USD file format used during caching.
- Make all USD dialogs use the same file formats list.

…che-file-type-persistence