Added FallbackImage property

[thewoodknight]

This PR lets you set a fallback bitmap for if an image binding is set to null. Using a Bitmap means its a bit easier to cache in memory rather than every failure creating a new Bitmap

I may very well have missed how to do this with the library already, so my apologies if its already there.

Usage

<asyncImageLoader:AdvancedImage
		Loader="{x:Static app:App.ImageLoaderInstance}"
		FallbackImage="{x:Static app:App.FallbackImage}"
		Source="{Binding Path=., Converter={StaticResource MetadataImageConverter}}" />

private static Bitmap fallbackImage;
public static Bitmap FallbackImage
{
    get
    {
        return fallbackImage ??= new Bitmap(AssetLoader.Open(new Uri("avares://WateryTart.Core/Assets/cover_dark.png")));
    }
}

FallbackBinding on Source only triggers when the binding itself fails, rather than returning empty/null strings.

[SKProCH]

Thanks!