refactoring: rename "one move back/forward" to "previous/next board position"
allow changing the board position in scoring mode
doc updates
additional board position navigation

change details
- add bar-like view with buttons to navigate the current game's
  board positions
- remove previous/next board position buttons from toolbar
- add a view next to the navigation bar view that displays
  information about the current board position
- tapping the current board position view switches between
  the navigation bar view and the scroll view that lists all
  board positions
- BoardPositionView now displays nothing for board position -1
- BoardPositionView can now handle repeated invocations of
  layoutSubviews()
- PlayViewController is now fully responsible for creating all
  board position related views (previously there was a mixup
  of reaponsibility with BoardPositionListViewController)
- new category on UIImage: UiImageAdditions
- move gradientImage class methods from UiUtilities to
  UiImageAdditions