Design improvements

[maxkatz6]

What does the pull request do?

1. Design.DataContext should support bindings.
2. Design.PreviewWith should work with any controls, as developer would decide previewing it.
3. Design.PreviewWith should work with IDataTemplate, but require ContentControl with Content data set.

Untested. Need to wait for the nightly builds.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055107-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[maxkatz6]

Design.PreviewWith should be a ITemplate<Control> property to make it work unfortunately.

[MrJul]

Public API for review:

namespace Avalonia.Controls
{
    public static class Design
    {
+        public static void SetPreviewWith(IDataTemplate target, Control? control);
+        public static Control? GetPreviewWith(IDataTemplate target);
    }
}

[maxkatz6]

@MrJul we can discuss another API option:

+        public static void SetPreviewWith(AvaloniaObject target, ITemplate<Control>? template)
+        public static void SetPreviewWith(ResourceDictionary target, ITemplate<Control>? template)
+        public static void SetPreviewWith(IDataTemplate target, ITemplate<Control>? template);
+        public static Control? GetPreviewWith(IDataTemplate target);

And additionally:

1. Find a way to force XAML compiler to use ITemplate overload fixing stackoverflow for user controls. OverloadResolutionPriority support when? :). But in this specific case we can just hardcode Design as a special case.
2. GetPreviewWith should always check if any template was saved for the control and use it instead of attached property.