Update Xamarin.Forms version to 5.0.0-pre3

[rookiejava]

Description of Change

This PR is to update the Xamarin.Forms version to 5.0.0-pre3. Also, this includes two major changes:

1. As you may know, since Xamarin.Forms 5.0.0, MediaElement related classes and interfaces provided in Core have been removed. (See [Core,iOS,Droid,UWP,Tizen] Remove MediaElement xamarin/Xamarin.Forms#12007 for more details). Accordingly, all classes and interfaces used in CircularUI for MediaView, MediaPlayer have been added again.
   (These are the classes and interfaces that were removed from Update Xamarin.Forms version to 4.6.0-pre4 #270.)

2. One of the big changes to the Xamarin.Forms 5.0.0 Tizen backend is the use of SkiaSharp to support shiny new XF5 features such as Brush, Path and Shape. So, Xamarin.Forms.Platform.Tizen.dll itself has a dependency on the SkiaSharp package. Accordingly, CicularUI didn't have to worry about the dependency of SkiaSharp anymore, and decided to include it in the CicularUI package without providing the CircularUI.Chart package that was provided as a preview anymore.

These two changes are big changes, but don't worry.
There is nothing to modify in your app to do this, and nothing changes in behavior.

Bugs Fixed

fixes #354

API Changes

The following classes and interfaces are migrated from Xamarin.Forms.Core.

Added:

• public class MediaSourceHandler
• public class EmbeddingControls
• public class FileMediaSource
• public class FIleMediaSourceConverter
• public interface IPlatformMediaPlayer
• public interface IVideoOutput
• public class MediaSource
• public class MediaSourceConverter
• public class UriMediaSource

Behavioral Changes

None