Generalize the EchoArea into a TransientMessage

[travitch]

Currently, there is an EchoArea abstraction in the core library. This is meant to represent transient messages like the minibuffer/echo area in emacs. The notion of a transient message makes sense in core, but doesn't need to be as specific as the EchoArea.

This change would also modify the EchoText message into a transient LoggingEvent, possibly with a timeout attached.