Adds SingleThreadedLogger to be able to keep a footer for where progress indication will go.

[coollog]

Part of #1297

This will be gradually improved once it is used in the LogEventHandler.

[chanseokoh]

Got some questions. Maybe due to not being able to see the big picture.

[chanseokoh]

Trying to understand why this needs to be a Runnable instead of a String. Isn't it that in the end what this Runnable will do is to write some string to plainLogger? Actually, what does "plain" mean here? Also, is it that this "message logger" (who will log a message to the console according to the Javadoc) is a different logger than the plain logger? (I didn't get what these two loggers will be in the end.)

[coollog]

So this is for the user to pass like (Maven) () -> log.info("log message") or other logging method, so that this class is independent of the actual different logging levels.

plain is for a logging method similar to just plain System.out.print, so it will be like lifecycle for Gradle and probably System.out.print for Maven (might need to do some things differently if wanting to use Maven info logging).

[chanseokoh]

Ah, now I see the intention. Basically, this class is the LogEvent handler (or will be used by whatever LogEvent handler) to "move" the footer (the multi-line progress display) down by one line after logging the message (if it gets printed to the screen, for example). This is done by calling this log() message. And at the same time, this class is the ProgressEvent handler (or will be used by whatever ProgressEvent handler) to update the progress display, which is done by setFooter(). I see where my confusion came from: first, the potential mix of LogEvent and ProgressEvent; secondly, the mix of two output channels, one for messageLogger and plainLogger (for example, one going out to a file and the other going out to a screen); and the messageLogger is basically an unrestricted lambda. Not that I am suggesting something concrete here... don't really have an idea.

[coollog]

Yea, this will be used by the LogEventHandler. A ProgressLogger will be the one that handles ProgressEvents and directly calls setFooter on this or set the footer via a LogEvent type of CHANGE_FOOTER or something.

[chanseokoh]

OK from my side. Renaming the class sounds good.

[chanseokoh]

LGTM, but I have one comment after you introduced shutdown.