Feature virtual categories subtree

[romainruaud]

This is a PR for #49

This is partially based on PR #227 so it may need to be refactored if we chose to not integrate this feature at all.

Working :

• navigating through subtree of a virtual category root on the catalog.
• this is not implemented on search results, since I "lost" the initial virtual category root while drilling through category filter : may be hard to do.
• products URLs are also properly displayed (if configured to be displayed with category path) under category root subtree.

Feel free to test and let met know.

[romainruaud]

I'll fix the build but can you review this one first to ensure the approach was correct ?

[afoucret]

This PR is outdated now.