Clean fakes option when purging orphans.

[ezragoss]

Just as an artist-friendly precaution I included an option to clean fake users flags on export so the dependency graph doesn't pull in unused items that depend on other items that depend on fake users.

I haven't looked a ton at the node export code so this is just for the non-node exports but if this is relevant to nodes as well its a simple add.

I've tested this with collection and object exports and it seems to work fine.

[Fweeb]

Conceptually, I like this idea. However, sculpting brushes are all datablocks with fake users. The code in your pull request clears the fake users these brushes, making them unavailable in the saved .blend file. That's not ideal... especially if the exported object is sculpted and the user intends to refine it further after export.

Maybe it makes sense to filter and exclude "native" datatypes like Brushes.

[ezragoss]

I was thinking something similar. The more I looked into this after opening this PR the more I think we can get sophisticated with the purge. This methodology of purging scenes prior to asset export has been something I've been implementing as a sidecar to this plugin for clients since I opened this PR and its saved a ton of time and money - so at least practically I can say it's desired in production! But agreed that we don't want to brute force the purge