I am currently building a Magento store. I have started to add all my categories, but I have so many sub-categories that the left navigation menu is getting too long. So I want to only show my main categories in the left navigation menu. I want a small + symbol next to each main category name which if clicked expands that category to show the list of sub categories. The url below is a Magento demo store I found which has the kind of thing I want.
[url removed, login to view]
I have seen a few existing Magento extensions for this feature but none of them are quite what I am looking for so would like this feature customized specifically for my store. Would like it done without changing any of the core Magento files so upgrading to later versions of Magento will not cause a problem.