Don't store data in home directory

Currently, DiffusionBee keeps generated images and other data in a `~/.diffusionbee` directory. It is annoying and bad practice to unnecessarily clutter my home directory with hidden folders. I believe the macOS standard would be to keep this sort of data in [the library](https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/MacOSXDirectories/MacOSXDirectories.html) in `~/Library/Application Support/DiffusionBee`. Especially since we already store some data there, it would make more sense to consolidate