add toolbar constants to UiElementMetrics
board is now correctly centered even if PlayView frame is rectangular
use toolbar as container for board navigation controls

Display navigation buttons, board position list view, and current
board position view inside a toolbar. Reason for this change: The
custom drawing previously used for rendering the navigation
buttons looked hideous and the touch area for each button was
much too small.

change details
- rename of one of the controller classes
- repositioning of status line and activity indicator because the
  new toolbar requires more vertical space
- the entire Play view is now uniformly filled with the board
  background color; this looks much cleaner now that there is
  almost no clear space left due to the toolbar taking up so much
  room
- clean code refactoring in PlayViewController to make
  makeControllerReadyForAction() look simpler
doc/comment updates
new button for discarding the current board position

- from the perspective of "moves" (which is how the user
  sees it), the button acts like a backspace button
- from the perspective of "board positions", the button
  acts like a delete button
- in a computer vs. human game, the button does not
  behave as "undo" did, i.e. it does not discard two moves
  at once; as a result it is possible that the last board
  position is one where it is the computer's turn to play;
  to account for this, the checks that prevent the user from
  making a move in this situation (panning, pass, play for me)
  are adjusted
change "play for me" button to "computer play" button

- when the user views a board position where it is the computer player's
  turn to move, the button formerly known as the "play for me" button can
  now be tapped to let the computer play for itself
- this lets us change the semantics of the button to the more general
  "computer play", which also happens to be a better fit for the button icon
- besides making the UI more intuitive, this change is also important
  because of the new button for discarding board positions that was introduced
  in the previous commit: in a game where the computer player makes the first
  move, discarding all board positions up to the very first would make it
  impossible to play any further moves (the computer would not start playing
  automatically, and tapping "play for me" would simply spew out an alert)
do not let the user make a move while it is the computer's turn
eliminate "cannot play on computer's turn" alert that is no longer used
update enabled state of board position navigation buttons