XForms Status Update

Checked in last week:
* <insert> and <delete> (#278211)
* fixed node binding context for controls (#280017)
Pretty essential issues.

This week will most likely include work on:
* support for pseudo-classes like :enabled, :disabled, :read-only, etc. (#271720)
* fix the last issue with :repeat-index/<setindex> (#278209)
* support for hasFeature() (#279057)
* issues with itemset (#278277)

One major issue is a problem with the asynchroneus loading of the external instances. Let's see if we'll get to that this week too.