Add template selectors to the editor

[hexawyz]

📝 Description of the Change

This adds DataTemplateSelector properties to NodifyEditor for each DataTemplate property.
See: #252

Testing ConnectionTemplateSelector here:

using System.Windows;
using System.Windows.Controls;

namespace Nodify.Playground
{
    public sealed class ConnectionTemplateSelector : DataTemplateSelector
    {
        public DataTemplate? Circle { get; set; }
        public DataTemplate? Triangle { get; set; }
        public DataTemplate? Square { get; set; }
        public DataTemplate? Default { get; set; }

        public override DataTemplate? SelectTemplate(object item, DependencyObject container)
        {
            if (item is ConnectionViewModel viewModel)
            {
                return viewModel.Input.Shape switch
                {
                    ConnectorShape.Circle => Circle,
                    ConnectorShape.Triangle => Triangle,
                    ConnectorShape.Square => Square,
                    _ => Default,
                };
            }
            return base.SelectTemplate(item, container);
        }
    }
}

        <local:ConnectionTemplateSelector x:Key="ConnectionTemplateSelector"
                                          Circle="{StaticResource ConnectionTemplate}"
                                          Triangle="{StaticResource CircuitConnectionTemplate}"
                                          Square="{StaticResource StepConnectionTemplate}" />

🐛 Possible Drawbacks

None that I know of.