Exclude the uid->wav_pool_file dict from project history...

Months ago I implemented a wav pool that only loads files that are actively
being used in the project, so there is no memory hit from doing this.
What this does solve is the project history adding an entry for every
item loaded in the wav pool, ie:  I loaded a .sfz file, it added 64
entries to the wav pool that would all probably have undesirable consequences
on a partial-undo...