* only expose necessary functions and types in header
* don't expose appender internals
* add generic function WLog_ConfigureAppender to have the possibility
to configure appender specific settings
* detect appender availability if WLog_SetLogAppenderType or
WLog_Appender_New return FALSE or NULL respectively the appender isn't
available or the initialization failed. This is very useful for the
use with optional appenders.
* add Free to the appender interface. At the time of the Free the
appender is known and available so it can be called directly (instead
of calling the right function according to the type)
* make all appender internal function static
* all appenders return the generic wLogAppender type now. Typecasts
are internally done where necessary this abstracts the appenders more
cleanly
