Add support for sections in listview

[BobDickinson]

Both Windows and iOS list views have a built-in concept of "sections" (where there are section headers for ranges of list view items). The simplest example of this is the contacts sample where there is a section header for each letter of the alphabet, where the contact entries starting with that letter show under that section header. The Azure mobile "field engineer" sample app also uses this functionality. Android doesn't have this concept built in, but there are several third party libraries/samples that implement this functionality (by just inserting appropriate rows in the view and treating them specially as section headers - they have their own style and are non-selectable). Xamamin offers this support (see their "Labelled Sections" sample) on Android.

This is moderately complex (the listview is already by far the most involved control). We would also need to figure out a clean way to do data binding using sections (where the section header binding was structurally related to the item binding, perhaps binding to a section, where the item binding context was then relative to the section binding).