pkgs.writeTextFile doesn't like spaces in filenames.
This is fixed in #162246, but other tools will probably dislike
spaces in .desktop file names as well, and that PR will take forever,
so let's fix this anyway.
This updates makeDesktopItem to explicitly support all the fields in the spec,
converts list-like fields to native Nix lists instead of semicolon-separated strings,
and allows automatically generating [Desktop Action] sections from Nix code
instead of hardcoding them as extraConfig strings.
- New parameter `extraDesktopEntries` to easily add some less usual entries to the desktop file
- Rewrite of the core logic. Instead of a key-value-list, use an attribute set with nullable values to make it overridable
- Added some comments
- Some cosmetic/readability code refactors
- I didn't like the doubly nested strings around the `fileValidation`
"Application" is deprecated, "Other" is invalid, there are no generic
categories, and the Categories fields is optional per the spec.
Fixes the defaults after #75729.
