GDA-5000 Switch to SLF4J 1.7.5
GDA-5000 / GDA-5780 / GDA-5771 Switch uk.ac.gda.swingclient to Logback 1.1.0

* Update PanelAppenderBase and subclasses following introduction of
  ILoggingEvent interface

* Add layout field to PanelAppenderBase because AppenderBase no longer has one