Fix: recursive folder search for slideshows

[JuanPBP03]

Fixes #102. Adds a config file that defaults to a recursive search (since thumbnails are searched recursively too). An additional UI toggle for this setting is in progress.

Also adds a fallback for XDG_DATA_DIRS to comply with XDG spec in case it is not set (as was the case in CachyOS for me) which also blocks the recursive branch.

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[nyabinary]

Shouldn't this be in draft status if the UI toggle isn't ready yet?

[jacobgkau]

Shouldn't this be in draft status if the UI toggle isn't ready yet?

Not necessarily. New settings can be added with config file support only, and later added to the GUI if the UX team approves.