Add Xamarin.Forms.Platform.Tizen project

[rookiejava]

Description of Change

This PR adds support for Tizen backend (Xamarin.Forms.Platform.Tizen) on Xamarin.Forms. All the controls are implemented expect OpenGLView, AppLink and MapView, but not all features are present. We plan to add UITest in the future (another PR).

Xamarin.Forms.Platform.Tizen proprect uses .NET Standard 1.6 TargetFramework temporarily. The target framework of it will be changed when the Tizen TFM is available in NuGet client.(NuGet/NuGet.Client#1083).

For this reason, unfortunately, the artifacts of this project (e.g. Xamarin.Forms.Platform.Tizen.dll,..) couldn't be included in Xamarin.Forms.nupkg in this time. By alternative way, the separated nuget package for this (Xamarin.Forms.Platform.Tizen.nupkg) has been uploaded to nuget.org.

For more details, please visit the https://developer.tizen.org/development/tizen-.net-preview/introduction.

API Changes (Xamarin.Form.Core)

Added:

• public const string Device.Tizen = "Tizen";
• Xamarin.Forms.PlatformConfiguration.TizenSpecific APIs are also addded.

Behavioral Changes

Add Tizen backend

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

[rookiejava]

This is from #761