Table object: add "Warning" to manually created warning portal messages.

This follows the Plone standard and matches better with themes styling
portal messages in general.

![bildschirmfoto 2013-09-13 um 10 15 42](https://f.cloud.github.com/assets/7469/1136765/b3232a16-1c4c-11e3-941e-18553f2d611f.png)