SLING-3341 - [logback] Race condition in configuring Logback

Fixing another race condition which occurs in LogbackManager constructor

-- Change in LogConfig to make categories unmodifiable
-- Change in LogPanel to render name of OSGi Appenders in a better way

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1562003 13f79535-47bb-0310-9956-ffa450edef68
SLING-3148 Set appropriate label for the configuration metatype

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1562005 13f79535-47bb-0310-9956-ffa450edef68