[Feature Request] Add a setting to set a custom output directory

It'd be really nice to be able to choose a custom output directory for generated images in settings. Currently, generated images are buried in a hidden folder which is less accessible to many and may not be that great of a place to put generated images. See #376.

Scenarios where this would make sense:

- Make the folder containing the generated images more accessible (less hidden)
- Would allow outputting files on an external drive
- Would allow outputting files in an iCloud-synced directory
- Would allow putting images beside your other media in a specific spot for apps like Plex or self-hosted gallery to index, for backup software to pick up, etc.

The directory could be what it is right now by default (to avoid a "breaking" change), but be customizable.