Suggestion: add intermediate class for sinks logging to a file.

[bartgol]

I have a piece of code that creates a file sink (either basic or rotating), stored in a base_sink pointer. When we have to query the name of the file, we need to perform a cast to basic_file_sink to get the filename, and if the cast fails, cast to rotating_file_sink. It might be convenient to have an intermediate class file_sink that simply adds the virtual method filename(). Then, one can store a file sink as a ptr to file_sink, and directly query for the filename. Or, if this sink is stored among other kind of sinks, dyn cast once to file_sink to get the file name.

[gabime]

Thanks. Need to consider this also for other common file operations(e.g. callbaks on file open, closed, etc. see #1225)