Make MarkerWrapper constructor public.

  - Needed in order to move other classes to org.apache.logging.slf4j

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1583064 13f79535-47bb-0310-9956-ffa450edef68
Move SLF4J impl classes where possible.

  - The remaining four need to stay where they are in the org.slf4j.* namespaces due to SLF4J SPI requirements.
  - SLF4JLogger also renamed to Log4j2Logger.
  - See issue LOG4J2-564.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1583065 13f79535-47bb-0310-9956-ffa450edef68