add GoGameDocument to support concept of document dirty flag

this is a preparation step for the upcoming commits related to issues #42
("Display "discard old game" question only if game has not been saved")
and #20 ("Open .sgf files received via email").
bump NSCoding version number
"discard game in progress" question is now based on document dirty flag (closes #42)
fix NSCoding support in GoMove (move number was incorrectly stored as object)
use NSCoding to backup/restore the game in progress (closes #114)
disable resigning while viewing an old board position (fixes #115)
BackupGameCommand now saves an .sgf file only when explicitly requested

currently all actors that use BackupGameCommand *do* want to save an .sgf file,
but future actors sometimes will not want this
save application state more frequently (together with user preferences)

this change fully leverages the previous changes to BackupGameCommand and
RestoreGameCommand (both of which now work with an NSCoding archive);
because we now save application state on application suspend (and a few other
occasions), RestoreGameCommand is now capable of restoring not only the board
position, but also the GoGameDocument properties and the scoring mode state
(the latter has never been possible before).
fix many issues related to the new NSCoding restore scheme