Click tag mentions

You can now click on a username in the main chat to add a "@username"
tag to the chat. Makes communication with other users using tags faster.

cockatrice/src/chatview.h

@@ -49,6 +49,7 @@ signals:
     void cardNameHovered(QString cardName);
     void showCardInfoPopup(QPoint pos, QString cardName);
     void deleteCardInfoPopup(QString cardName);
+    void addMentionTag(QString mentionTag);
 };
 
 #endif