[+FEATURE] Fluid (ViewHelpers): Add render mode to FlashMessage ViewHelper

Since r3062 Extbase uses the t3lib_FlashMessageQueue to store flashmessages.
Now you can output them including title and severity, making it possible
to render flashmessages just like the ones from the core in BE mode.
To keep backwards compatibility, the default rendering still outputs
only the message bodies in an unordered list. Set the renderMode argument
to "div" in order to display the new style:
<f:flashMessages renderMode="div" />

Relates to: #10821