Move attributes to src/core/attributes subfolder (ABI break)

Same as with models and jobs. Modernizes and cleans up the code a little.
No changes needed to client code except for recompiling.
1 job for master in 30 minutes and 5 seconds (queued for 12 minutes and 9 seconds)
Status Job ID Name Coverage
  Build
passed #61837
linux kf5-qt5 qt5.12

00:30:05

1.0%