Group items handling with functions is broken

[splatch]

Earlier when initial split of openhab core happened handling of group items was left for later. The later have come and I had to take care of it.

The initial design assumptions which we had expected thatItemFactory will be dynamic and not fixed to only "core" items. Same argument was made for functions.
The group functions handling does not have enough of traction (yet) to move it forward. Looking at it we will probably need first to update DTOs and how these are handled. Only then we will be able to detach actual function implementation from core.