no longer save app state when app is suspended (reverts 775f3e3)

the change made in 775f3e3 was FATALLY wrong: application state must not be saved during
application suspend because the application might be in an inconsistent state at this time.

for instance, scoring might be in progress (this was actually what made me notice the
problem), or GTP command that belongs to a larger, unfinished operation might be
processed at the moment.

this case shows that the concept for saving application state is fragile and needs to
be redesigned so that it is more resilient against implementation changes; if the
developer needs to constantly think HARD whether or not is safe to save state at
this moment and place in the code, something is guaranteed to go wrong again in
the future. I have added issue #124 to address the problem.

finally, the consequence of reverting 775f3e3 is that some application state
properties (board position, GoGameDocument properties, scoring mode state)
are not saved as often as possible. instead they are saved only after changes
that also trigger the .sgf file to be saved.
no longer write/read GoScore to/from NSCoding archive during backup/restore

restoring scoring mode from the NSCoding archive does not work if the archive is created
while scoring is in progress. this is exactly what happens, though: discarding a move while
in scoring mode 1) triggers a new round of scoring, then 2) makes a backup. if the backup
occurs while scoring is still in progress (ALWAYS happens when the GTP engine is finding
dead stones), the resulting NSCoding archive contains a GoScore object that is still in the
middle of calculating the score.

instead of implementing a clunky fix for the problem in GoScore, we don't try to restore
scoring mode at all. a real and better fix for the problem should hopefully emerge when
#124 is implemented in the next version.
retain current board position by saving the app state after each board position change

in ffeaf54 we removed the property that could be used to save/restore the current board position;
in 96b0eeb we had to remove the ability to save the app state on app suspension due to a fatal
flaw of the concept. as a result we are no longer able to restore the current board position after
an app launch. this is unacceptable because the user had this ability in the previous version of
the app, so the change in this commit is implemented as a temporary workaround.

the consequence is that changing the board position now takes a lot more time. for instance, on
an iPhone 3GS the backup takes about 1 second, on an iPad 2 it is about 0.3 seconds. although
this solution is rather unfortunate, I chose to implement it because it is only temporary and can be
reverted after #124 has been implemented in the next version.
bugfix: remove NSCoding archive after new game is started

this fixes the following scenario:
1) start new game + play a few moves
2) start another new game
3) suspend + kill the app
4) relaunch the app

- expected: app presents a new game created in 2)
- actual: app restores game state from 1) because NSCoding archive is still present
fix display of progress HUD in SetupApplicationCommand
remove NSCoding archive file if restoring from it fails
add explanatory footer messages for several Play View user preferences