Don't destroy formatting when editing previous messages

Adds a few new methods to grab the markdown/slightly rich text from the
message, and will intelligently re-insert user mentions as needed.

src/chatbarcache.h

@@ -5,8 +5,11 @@
 
 #include <QObject>
 #include <QQmlEngine>
+#include <QQuickTextDocument>
 #include <QTextCursor>
 
+class ChatDocumentHandler;
+
 /**
  * @brief Defines a user mention in the current chat or edit text.
  */
@@ -174,6 +177,11 @@ public:
      */
     QList<Mention> *mentions();
 
+    /**
+     * @brief Update the mentions in @p document when editing a message.
+     */
+    Q_INVOKABLE void updateMentions(QQuickTextDocument *document, ChatDocumentHandler *documentHandler);
+
     /**
      * @brief Get the saved chat bar text.
      */