Switched to 'enum class' for the LogMessageType enum
Added functions for accessing binary/text content of RawConversationFile's
Added the ErrorContext facility

This allows us to easily append context information to all errors, warnings
etc. occuring in a given scope.
Improved error context strings
Added error detection to RawConversatiinFile::allUtf8Text() function
Pidgin Text Format decoder: read and parse conversation header
Updated Pidgin format research and design notes

I now have confirmation that old versions of Pidgin store local time
without converting it to UTC, which means that the timezone has to be
guessed.