[2332]Made logging level switchable via preferences. Added link to wiki

I am not sure whether it is a good idea to add LogbackUtils.java to this plugin.
I did not put into ch.elexis.core.logging because this plugin is by design not references by ch.elexis.core, as it should be completly optional to allow other slf4j compatible logging solutions.
Therefore I am open for suggestions for renaming and/or adding a new plugin/fragment.