Add ordered adapter preferences

[mjcheetham]

Applications may depend on capabilities that are only available from a particular WebView implementation, but backend-specific switches do not scale as more adapters and hosting modes are added.

Expose a common ordered preference list on NativeWebView and NativeWebDialog. A non-empty list is exhaustive, allowing callers to express fallback order or require a single adapter, while an empty list preserves the existing platform defaults.

Apply selection to WebView1 and WebView2 on Windows and to WPE WebKit and WebKitGTK on Linux. This also lets authentication callers select an adapter through the existing NativeWebDialogFactory without adding WebView-specific options to WebAuthenticatorOptions.

Example with NativeWebDialog

var dialog = new NativeWebDialog
{
    Source = new Uri("https://example.com"),
    AdapterPreference =
    [
        WebViewAdapterType.WebView1, // prefer WebView1 over WebView2
        WebViewAdapterType.WebView2,
    ]
};

dialog.Show();

Example with WebAuthenticationBroker

var options = new WebAuthenticatorOptions(requestUri, redirectUri)
{
    Mode = WebAuthenticatorMode.NativeWebDialog,
    NativeWebDialogFactory = () => new NativeWebDialog
    {
        AdapterPreference = [WebViewAdapterType.WebView1] // WebView1-only
    }
};

var result = await WebAuthenticationBroker.AuthenticateAsync(owner, options)

Example with NativeWebView via code+XAML

public IReadOnlyList<WebViewAdapterType> MyAdapters { get; } =
[
    WebViewAdapterType.WebView1,
    WebViewAdapterType.WebView2
];

<NativeWebView
    Source="https://example.com"
    AdapterPreference="{Binding MyAdapters}" />

Example with NativeWebView via XAML-only

xmlns:platform="clr-namespace:Avalonia.Platform;assembly=Avalonia.Controls.WebView"

<NativeWebView>
    <NativeWebView.AdapterPreference>
        <x:Array Type="platform:WebViewAdapterType">
            <platform:WebViewAdapterType>WebView1</platform:WebViewAdapterType>
            <platform:WebViewAdapterType>WebView2</platform:WebViewAdapterType>
        </x:Array>
    </NativeWebView.AdapterPreference>
</NativeWebView>

This is a proposal to address #16

[maxkatz6]

Can you mark PreferWebKitGtkInstead obsolete with a hint to use new API?

[maxkatz6]

PreferWebView1Instead can be completely removed and this code simplified. It wasn't part of the Public API.

[maxkatz6]

XAML compiler should be able to handle these enums as in AdapterPreference="WebView1, WebView2" syntax. But I don't remember if this property needs to be a mutable list or not.