Debugger: Make CliContext use BLooper for message passing

This commit replaces the custom implementation of message passing with
BLooper.

This commit does not fully transition the code to the BLooper paradigm.
WaitForEvent still exists, when, ideally, message passing should be
used to notify the waiting class that the results are ready.

Change-Id: Id223b9fc0ac1aa9a5a5aeb4af63da04f2cc7574b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6400
Reviewed-by: John Scipione <jscipione@gmail.com>
Reviewed-by: Rene Gollent <rene@gollent.com>

Debugger: Move CLI commands into their own subdir.

Debugger: Move CLI commands into their own subdir.