Refactor roster item positioning

The following discussion from !860 (merged) should be addressed:

• @lnj started a discussion:

  I think this function has two different use-cases. Can you make two separate functions out of it?

  • positionToInsert(RosterItem) -> int
  • newItemPosition(int currentIndex) -> int

  Maybe you can also avoid the newPosition - 1 above by doing that in the function then?

  And is it correct that beginMoveRows() always uses newPosition but m_items.move() uses newPosition - 1 in some cases?