Add severity filter to b:growl and b:messages

[jepsar]

I've opted for a serverity attribute on growl and messages:

<attribute>
  <description><![CDATA[Comma separated list of lower case severity names of which messages should be rendered in this component. Allowed severity names "info", "warn", "error", "fatal". Default value: "info,warn,error,fatal" (all severities).]]></description>
  <name>severity</name>
  <required>false</required>
  <type>java.lang.String</type>
</attribute>

This makes migrating from PrimeFaces easy.

[stephanrauh]

Thanks! You've even thought about adding the attribute to the *.jsfdsl file. Awesome!

Currently, we're a bit busy, so it may take some until we evaluate and merge (or - albeit unlikely - reject) your PR. Please stay tuned!