Remove 'Show offline contacts' Preference

To avoid having a cluttered chats-list with lots of contacts that are offline for long, instead of toggling a show/hide option, one can use 'leave chat' from the sub-menu to remove them from the list while keeping the chat-log. The conversations can be picked again from the 'Start new chat' dialog, and of course, they re-appear in the chats-list as soon as the chat-partner sends a message.