Change chat bar text field back to plain text

This was changed in 9ed5224470 but this
ended up causing all sorts of strange formatting issues like the quick
format bar not working, and pastes from other applications that kept the
formatting.

src/libneochat/chatdocumenthandler.cpp

@@ -330,7 +330,7 @@ QString ChatDocumentHandler::getText() const
         qCWarning(ChatDocumentHandling) << "getText called with no QQuickTextDocument available.";
         return {};
     }
-    return document()->toRawText();
+    return document()->toPlainText();
 }
 
 void ChatDocumentHandler::pushMention(const Mention mention) const