libcommon|SaveInfo: Track status changes of saved game sessions in SaveInfo

SaveInfo now monitors the status of the saved game session internally
and notifies interested parties when it changes. Note that the saved
session file is not currently monitored, however. This means that one
must still prompt a manual updateFromFile().

Also, changes to the user description are similarly observable.
libhexen|ACS|Cleanup: Applied 'session' naming convention to ACScript
Fixed|libcommon: Parsing of episode number from map Uris (refactoring oversight)

By default String::toInt() does not tolerate suffixes.