Adding Existing .xaml files

Ported from https://github.com/dotnet/roslyn/issues/11339#issuecomment-248950756

---

Version Used: VS 2015 u2

Steps to Reproduce:
1. Make a new solution(Blank Xamarin.Forms Portable)
2.Click on the PCL project, and select add Existing item
3.Select any .xaml files you have that are related to xamarin
4.If you add multiple .xaml files, their backing code .cs, will not get included.
5.if you add the .xaml file, one by one, the backing code .cs will be included, but its properties are:

Build Action: Page
Custom Tool: MSBuild:Compile
which should not be the default values.

Expected Behavior:
It should add multiple .xaml files, and include their backing code .cs file.
also, it should set the .xaml properties as:
Build Action: Embedded Resource
Custom Tool: MSBuild:UpdateDesignTimeXaml

Actual Behavior:
It adds multiple selected .xaml files without their backing .cs file.
and if you add .xaml file, one by one, it makes its properties as:
Build Action: Page
Custom Tool: MSBuild:Compile