Load items based on scroll event and lastVisibleIndex property
Load items based on scroll event and lastVisibleIndex property

Loading more items when the "loading..." cell is resolved has some
disadvantages:

- loading starts not before the dummy "loading ..." item is resolved,
  i.e. not before scrolled to the end of the list
- loading depends on the client's buffer size
- the dummy item cannot be reused, it has to be replaced with another
  dummy item to ensure another change:item event is triggered

Change-Id: I1b655cc6bd7b6a5e3152529b1fbed1b673aaceb1

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js