Make HasMenuCurrent return true for $sectionMenuEntry.IsAncestor($pageMenuEntry)

[bep]

We currently have the sectionPagesMenu setting which in a roundabout way does the above, but it's rather clumsy and I suspect added before we got a proper data structure in this area. For one thing, it makes it hard for themes to make it working out-of-the-box.

I think we can retire that setting by being a little bit smarter about it, that is:

• We always try the very best to attach a page to a menu item (even if defined in config.toml)
• For menu entries that is defined for a section, any page below that section will (by default, I'll think of a way to turn it off), return true for HasMenuCurrent.