Added number of placed ships check.

Previously, the program allowed user to get into the game, even
if not all ships were placed on the field. Now it should not
happen.
Added ability to clear layout on Arrangement Screen.

Previously, user could not alter the position of the ships that
have already been placed. Now this option is available via clearing
the whole layout.
Added undo action on Arrangement Screen.

Now user is able to undo ship placement, in reversed order
of their placement.