MessageEventModel plain text role

Update the message role to be named plainText and use it the message delegate for openMessageContext.
This removes the need for plainText in controller so it is removed.

src/controller.h

@@ -207,14 +207,6 @@ public:
      */
     Q_INVOKABLE QString formatByteSize(double size, int precision = 1) const;
 
-    /**
-     * @brief Return a plain text representation of the text in a given QQuickTextDocument.
-     *
-     * TODO: This is only used in messageDelegate to get the plain text of a message.
-     *       This should be changed to a role in messageeventmodel via texthandler.
-     */
-    Q_INVOKABLE QString plainText(QQuickTextDocument *document) const;
-
     /**
      * @brief Force a QQuickTextDocument to refresh when images are loaded.
      *