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: Split into core library and application.

- Add subfolder src/kits/debugger which contains the debugger's core
functionality and lower layers. Correspondingly add headers/private/debugger
for shared headers to be used by clients such as the Debugger application
and eventual remote_debug_server. Adjust various files to account for
differences as a result of the split and moves.
- Add libdebugger.so to minimal Jamfile.

Debugger: Move CLI commands into their own subdir.

Debugger: Move CLI commands into their own subdir.