Undo no longer works with multiple files

[Sappharad]

When background processing was cleaned up in 2.3, the undo feature broke and it stops processing after the first file(s). The number of files depends on how many threads you were using, 4 is the most common default for most machines. Bug reported last month via email.

[Sappharad]

This was introduced when I attempted to clean up memory management to prevent leaks. Undo was never updated with the same changes as Analyze and Apply, so after the first file gets done processing it tries to remove it from the common pending files list but there was no pending files list to remove it from.