Spring-loaded folders

I've ported the old 2.32 patch from Nautilus' (now-hidden) bug 44001, that implements spring-loaded folders, to Caja. It changes the internal API a little bit (adding a parameter to caja_application_get_spatial_window ()), which I've accounted for, but it's otherwise pretty low-impact. 

The behavior in navigational mode is perhaps not the best, but I don't use it myself, so I'm not sure what would work best with most peoples' workflow.