Improve reduce project

[Shebbe]

Sometimes with complex/large projects we have a desire to clean up stuff that has been floating around from testing phases. Typically the best way to clean everything up is to reduce the project based on selected comps.

What often becomes an issue is that we can't do this if we want to maintain our global folder structure because it deletes any folder that doesn't have contents.

Would be nice to have an option to perserve folders and then clean up the ones you actually don't need manually.

Another big hurdle with reduce project is that expression that reference comps that aren't used anywhere in the selection for reduction get deleted so they break. At the minimum I would expect a warning (prior to reducing), but even better would be to simply preserve them by default.

So:

- Option to keep all folders

- Default: Preserve comps that are referenced via expressions

[Warren Heaton]

Some great recommendations.

Even though we can undo the results, I'd add being able to preview the reduction before commiting to the changes with an option to show items being deleted so they can be added to what is being kept. 

I currently add everything I want to keep to a temporary containing Comp before doing a reduce and while that works well enough, it would be great to see improvements.