[renamerOnUpdate] Element feature requests

[echo6ix]

Four element field feature requests for renamerOnUpdate:

1. $filename - The incumbent filename of the file. Example use-case: To keep the existing file name as-is, but change file path location only.
2. $stashid_scene - StashID of the scene
3. $stashid_performer - StashID of the performer(s)
4. $studio_code - The new scene studio code added in recent develop versions