[Spec] ImageSourceHandler

# [ImageSource]

Create a service that handles the ImageSource logic on shared code based [on this discussion](https://twitter.com/mattleibow/status/1302407836748611586) between me, @PureWeen, and @mattleibow.

The general idea is to have an ImageSource handler class that will handle all ImageSource updates in the shared layer and reflects them to the Control.

# API

## [ IImageSourceHandler]

```csharp
interface IImageSourceHandler
{
ImageSource Current {get;}
ImageSource LoadingPlaceholder {get;}
ImageSource ErrorPlaceholder {get;}
ImageSource Desired {get;}

// Those can be internals

Action OnLoadingStarted {get;}
Action OnSourceChanged {get;}
Action OnLoadingFailed {get;}
Action OnLoadingCompleted {get;}
}
```

### Properties

| API | Description |
| ------------- | ------------- |
|Current | This will be the current ImageSource for displayed Image. |
|LoadingPlaceholder | ImageSource that will be presented while we load/download the Desired ImageSource |
|ErrorPlaceholder | ImageSource that will be presented if something goes wrong with the Desired ImageSource |
|Desired | The ImageSource that the user wants to present in the control |

### Events

| API | Description |
| ------------- | ------------- |
|OnLoadingStarted | Action that will be fired when we start to load the Desired ImageSource |
|OnSourceChanged | Action that will be fired when Current value changes |
|OnLoadingFailed | Action that will be fired when something goes wrong during the Loading process |
|OnLoadingCompleted | Action that will be fired when we loaded the Desired ImageSource successfully |

# Scenarios
```xml

```

# Backward Compatibility

Minimum API levels?
Breaking changes?
Unsupported platforms?

# Difficulty: medium