Add favorites placeholder to Kicker's rootModel

Adds showFavoritesPlaceholder which is an empty placeholder but it allows to detect itself("KICKER_FAVORITES_MODEL"), thus allowing consuming side to use a custom page, which you'd want anyway(to allow dragging favs and stuff)

So a bit of context. With !371 (closed) I wanted to go with the third approach, so I built on further with !377 (closed), which turned out overly complicated, so instead of trying to be clever about it, I decided to go with a simpler approach

Supersedes !371 (closed) and !377 (closed)

See plasma-desktop!186 (closed) for reference implementation (latest commit)