Default to showing offline contacts

Many messengers dont show the status anymore, its not evident to new
users that most of their roster is hidden per default and they have to
show it to access the contacts

gajim/common/config.py

@@ -79,7 +79,7 @@ class Config:
         'autopopupaway': [opt_bool, False],
         'autopopup_chat_opened': [opt_bool, False, _('Show desktop notification even when a chat window is opened for this contact and does not have focus')],
         'sounddnd': [opt_bool, False, _('Play sound when user is busy')],
-        'showoffline': [opt_bool, False],
+        'showoffline': [opt_bool, True],
         'show_only_chat_and_online': [opt_bool, False, _('Show only online and free for chat contacts in roster.')],
         'show_transports_group': [opt_bool, True],
         'autoaway': [opt_bool, True],