logging interface extension for debug logging

As discussed in #122, I've extended the standard logging infrastructure with a new ILog.OnDebug method.

@Formator, this way You could implement the Common.logging solution as implementing a new CommonLog class based on the new ILog interface, what You've started at #142,
but the classic logging infrastructure remains, too.

I've made obsolete the DebugFileLogPath config setting, and all related codes, and replaced this hard-coded FileLog with standard ILog.

I've removed debug logging related FIXME comments, and replaced the temporary logging solution with ILog based logging everywhere.

I've implemented extra debug logging in various places, eg. exception details are logged into debuglog, and incorrect datatype messages are extended in DataDictionary, too.