[Xaml] Support native view declaration in Xaml

[StephaneDelcroix]

Description of Change

With this PR, the following Xaml is valid, and works as expected:

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:ios="clr-namespace:UIKit;assembly=Xamarin.iOS;targetPlatform=iOS"
             xmlns:android="clr-namespace:Android.Views;assembly=Mono.Android;targetPlatform=Android"
             x:Class="Xamarin.Forms.Controls.XamlNativeViews">
    <ContentPage.Content>
        <ios:UILabel Text="I'm Native" />
        <android:TextView  Text="I'm Native too" />
    </ContentPage.Content>
</ContentPage>

That means 2 things:

• native views are automagically wrapped into the appropriate wrapper
• xmlns defined on a non-matching targetPlatform (see TargetPlatform enumeration) are ignored

Bugs Fixed

None

API Changes

None

Behavioral Changes

None

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Consolidate commits as makes sense

[StephaneDelcroix]

Do NOT merge yet for the following reasons:

• It doesn't work as expected as there's a bug in our implementation of mc:Ignorable
• missing implementation for Android
• missing implementation for Windows

[StephaneDelcroix]

No change in this, unnecessary diff

[StephaneDelcroix]

If you review this, review only the 2nd commit. the first is part of another PR

[StephaneDelcroix]

notice the targetPlatform on these 4 lines. notice the cumbersome assembly name for uwp as well, but that's required to get it working