allow editing players and profiles from the Game Info screen (#122)
active profile is now a property instead of a method in GtpEngineProfileModel

this allows clients to use KVO; this is preparation step for further changes in
relation to #122
make controllers aware of changes to player and profile objects

GameInfoViewController and PlayerProfileSettingsController can be visible at the same time,
and both controllers can initiate changes to player and profile objects that are tied to the
current GoGame. as a consequence both controllers must observe these player and profile
objects so that they can update themselves when a change occurs.

this commit completes the functional changes required to implement #122.
immediately apply new profile when the active computer player's profile changes (#123)
active profile can no longer be deleted

this simplifies handling of the active profile at every corner; as a trade-off the
active profile can be changed in mid-game (see previous commit); this commit
relates to both #122 and #123.
fix crash when creating new GTP engine profile