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Perhaps creating new activity from directory could be good idea. You go in dolphin to directory with you project and click create activity for current directory.

What happens?

For each file type (graphic, text, music, etc.) could be created plasmoids, grouped by grouping plasmoid (each image plasmoid will be put into image grouping plasmoid, text plasmoid into notes grouping plasmoid, etc.) and folderview with path to project directory will be created.

Also, we could create many folderview, each to filter different file types: program code, README-COPYRIGHT-INSTALL-LICENSE, data, graphics, etc.

Interesting idea, I like it!
But certainly that automatic creation of plasmoids and playlists needs to have a dynamic character, since in the end a "project" typically is something that creates new things...