Fix slow progress updating for wimsplit

Fix slow progress updating for wimsplit

wimsplit only prints a progress message when starting each WIM part.
That could be very infrequently since each part could be gigabytes.

Fix it to update the progress regularly as data is written, like the
other wimlib-imagex commands do.

This required changing the library to report
WIMLIB_PROGRESS_MSG_WRITE_STREAMS messages from wimlib_split() and
include the completed compressed size in them.

Reported at https://www.reddit.com/r/pcmasterrace/comments/hagu4k/wimlibimagex_split_stuck_at_0