Restructure solution a bit

src/Spectre.Console/Widgets/PanelHeader.cs

@@ -0,0 +1,60 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Represents a header.
+    /// </summary>
+    public sealed class PanelHeader : IAlignable
+    {
+        /// <summary>
+        /// Gets the header text.
+        /// </summary>
+        public string Text { get; }
+
+        /// <summary>
+        /// Gets or sets the header style.
+        /// </summary>
+        public Style? Style { get; set; }
+
+        /// <summary>
+        /// Gets or sets the header alignment.
+        /// </summary>
+        public Justify? Alignment { get; set; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="PanelHeader"/> class.
+        /// </summary>
+        /// <param name="text">The header text.</param>
+        /// <param name="style">The header style.</param>
+        /// <param name="alignment">The header alignment.</param>
+        public PanelHeader(string text, Style? style = null, Justify? alignment = null)
+        {
+            Text = text ?? throw new ArgumentNullException(nameof(text));
+            Style = style;
+            Alignment = alignment;
+        }
+
+        /// <summary>
+        /// Sets the header style.
+        /// </summary>
+        /// <param name="style">The header style.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public PanelHeader SetStyle(Style? style)
+        {
+            Style = style ?? Style.Plain;
+            return this;
+        }
+
+        /// <summary>
+        /// Sets the header alignment.
+        /// </summary>
+        /// <param name="alignment">The header alignment.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public PanelHeader SetAlignment(Justify alignment)
+        {
+            Alignment = alignment;
+            return this;
+        }
+    }
+}