Issue: XF2 Widgets: Area tree widget

A couple of widget-related requests for XF2. I'm posting this now while I remember the details, but do understand that new features are a low priority at this moment.

2) New widget: Wiki: Area tree.

This widget could just take the list of top-level Areas and display them in a clickable list. A bit like how XFMG has done its category navigation: https://xenforo.com/community/media/ (but without the added complexity of counting items in each area).

Child areas in a collapsed list would be a welcomed feature too, even if this implemented later into 4.1's development. Following XFMG's approach, sub-areas would be hidden in collapsed lists except if the currently viewed area has sub-areas and then only that list would be expanded by default.

1) New widget positions: Wiki Index: Sidebar/Sidenav/Header/Footer.

These would be the same as the standard Wiki positions, but only apply to the page identified as the index. The current wiki widget positions would no longer apply to the index page, but still apply elsewhere. If an admin wanted a widget to show in the sidebar of all wiki pages, then they'd just check both Wiki Index: Sidebar and Wiki: Sidebar positions.

Widget positions suggestion retracted - VW Area manager has similar functionality.