Updates to FormItem label and A11y (#395)

* feat: 1. add AccessKey support for form item.
2. Support label positioning for different content height.
3. polish demo.

* feat: add a transparent background for label.

src/Ursa.Themes.Semi/Converters/FormContentHeightToAlignmentConverter.cs

@@ -0,0 +1,36 @@
+using System.Globalization;
+using Avalonia.Controls;
+using Avalonia.Data.Converters;
+using Avalonia.Layout;
+
+namespace Ursa.Themes.Semi.Converters;
+
+public class FormContentHeightToAlignmentConverter: IValueConverter
+{
+    public static FormContentHeightToAlignmentConverter Instance = new(32);
+    public double Threshold { get; set; }
+
+    public FormContentHeightToAlignmentConverter()
+    {
+        Threshold = 32;
+    }
+    
+    // ReSharper disable once ConvertToPrimaryConstructor
+    // Justification: need to keep the default constructor for XAML
+    public FormContentHeightToAlignmentConverter(double threshold)
+    {
+        Threshold = threshold;
+    }
+    
+    
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if(value is not double d) return VerticalAlignment.Center;
+        return d > Threshold ? VerticalAlignment.Top : VerticalAlignment.Center;
+    }
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        throw new NotImplementedException();
+    }
+}

src/Ursa.Themes.Semi/Converters/FormContentHeightToMarginConverter.cs

@@ -0,0 +1,34 @@
+using System.Globalization;
+using Avalonia;
+using Avalonia.Data.Converters;
+
+namespace Ursa.Themes.Semi.Converters;
+
+public class FormContentHeightToMarginConverter: IValueConverter
+{
+    public static FormContentHeightToMarginConverter Instance = new();
+    public double Threshold { get; set; }
+
+    public FormContentHeightToMarginConverter()
+    {
+        Threshold = 32;
+    }
+    
+    // ReSharper disable once ConvertToPrimaryConstructor
+    // Justification: need to keep the default constructor for XAML
+    public FormContentHeightToMarginConverter(double threshold)
+    {
+        Threshold = threshold;
+    }
+    
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if(value is not double d) return new Thickness(0);
+        return d > Threshold ? new Thickness(0, 8, 8, 0) : new Thickness(0, 0, 8, 0);
+    }
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        throw new NotImplementedException();
+    }
+}