Debugger: Fix #14375, #12343.

TeamDebugger:
- When a job is aborted, instead of calling into the user interface directly
to reset the status message, post a message to do so. Also, only post the
message if we aren't already in a terminating state. Otherwise, if jobs
were still running while the team debugger is executing its destructor, it
would attempt to make calls to the already destroyed user interface. This
bug has likely been with us for quite some time, but was hidden by incorrect
ref counting in the past (see #12343).

Debugger: Cleanup.

- Split MessageCodes.h into a second file that separates out the
application-specific message codes from those used by the core.
Adjust includes accordingly. No functional change.

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.