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.cpp

@@ -670,11 +670,6 @@ bool Controller::hasWindowSystem() const
 #endif
 }
 
-QString Controller::plainText(QQuickTextDocument *document) const
-{
-    return document->textDocument()->toPlainText();
-}
-
 bool Controller::encryptionSupported() const
 {
 #ifdef QUOTIENT_07