Debugger: Add signals to DebuggerItemManager

Change-Id: Icbd14a683d1a5ec53aa2d2337d43a86c26bccac9
Reviewed-by: hjk <hjk121@nokiamail.com>
Debugger: Rewrite logic to use signals on DebuggerItemManager

Change-Id: I82e1cd3cca9cc2d79366e1af054640dadebf7871
Reviewed-by: hjk <hjk121@nokiamail.com>