Tree: restore the mechanism for highlighting network items
Refactor TreeHeader out of TreeDelegate
Refactor TreeBadge out of TreeDelegate
Refactor TreeSpinner out of TreeItem
Themable TreeIndicator
Remember last highlight
Minimalistic formatting for numeric messages
MessageFormatter: use IrcInviteMessage::isReply()
Require libcommuni 3.3
Fix alert for visible documents but inactive window
OSX: fix the top margin of the user listview
Merged events: distinguish abrupt disconnection from quit
Tree: remember last seen messages for badges

Don't increase badge numbers for already seen messages (bouncer users).
Tweak the tree indicator
Tweak the tree indicator on Windows
Fix last seen messages stamp for tree badges
Reset connection error highlight upon successful connection
Avoid wrapping in event tooltips
Pure plugin interfaces
Fix timestamp for previously seen alerts

Use the same technique than for latest seen message stamps; compare
to the timestamp from _previous session_ to avoid collecting a new
timestamp on startup and thus missing an alert from backlog.