OP NOTE: this was the original right answer and in some ways still is a good and for some best answer including me. BUT this answers the original question best I think.
Visual Studio 2015 finally adds support for regions in XAML. You enclose your regional code like this:
<!--#region RegionName-->
All Your XAML In Here
<!--#endregion-->
This will add a collapse icon to the left of starting line, using which you can expand/collapse the region.
Don’t know how handy this would practically be since we can already collapse/expand any node in XAML editor. It’s there anyway if you want to use.