After a long course of implementations and reimplementations, this is coming close to become finished.
The favourites are based on KAStats (current master). It allows favourites to be set to all activities, or the user can choose which activities to show a specific favourite application on.
This change covers applications, files and contacts, other favourites are still based on the old model (now named SimpleFavoritesModel).
{F1028047}
This patch requires KF 5.30. This probably means it will have to wait for Plasma 5.10 since 5.9 will not depend on 5.30.