Right now we check if the user is logged into the Steam account. This fails if the user has set Steam to "offline mode".

We should allow the game to played in this situation, but we need to consider at least the following:

- achievements that are unlocked in offline mode need to be unlocked in Steam once the user is online again
- the number of users in "crowd mode" will be reset to zero, which is probably ok
- some Steam functions can't be used, e.g. the "server time"