steam-friend: implemented a per buddy data container

This removes the unclean implementation for managing friendship
relationships, but most importantly expands capabilities for future
implementations.
steam: allow game play statuses to be output to account channels

This makes it possible to have an action message sent to the account
channels whenever a friend's game play status is changed. A new setting
has been introduced for this feature: game_status. By default,
game_status is set to false disabling this feature.