Add WIMLIB_PROGRESS_MSG_EXTRACT_{FILE_STRUCTURE,METADATA}

Add WIMLIB_PROGRESS_MSG_EXTRACT_{FILE_STRUCTURE,METADATA}

These are essentially updated versions of
WIMLIB_PROGRESS_MSG_EXTRACT_{DIR_STRUCTURE,TIMESTAMPS} which were removed
in v1.7.0.  The new versions will file periodically, so they can be used
as cancellation points where the library user provides
WIMLIB_PROGRESS_STATUS_ABORT.