The intend of WindowIterator is to support users who need to iterate multiple windows. It keeps track of the position and loads the next window if needed so that the user does not have to interact with the position at all.
Also remove the Window methods to get the first/last position and enforce the index based variant.
Update the documentation to make use of the newly introduced API.
See: #2151
Original Pull Request: #2787
0c0c1afc8e