Fix tree rendering

Fixes some tree rendering problems where lines were not properly drawn
at some levels during some circumstances.

* Change the API back to only allow one root.
* Now uses a stack based approach to rendering instead of recursion.
* Removes the need for measuring the whole tree in advance.
  Leave this up to each child to render.

examples/Console/Prompt/Program.cs

@@ -1,4 +1,3 @@
-using System.Collections.Generic;
 using Spectre.Console;
 
 namespace Cursor

examples/Console/Trees/Program.cs

@@ -6,42 +6,40 @@ namespace TableExample
     {
         public static void Main()
         {
-            var tree = new Tree();
-
-            tree.AddNode(new FigletText("Dec 2020"));
-            tree.AddNode("[link]Click to go to summary[/]");
-
-            // Add the calendar nodes
-            tree.AddNode("[blue]Calendar[/]",
-                node => node.AddNode(
-                    new Calendar(2020, 12)
-                        .AddCalendarEvent(2020, 12, 12)
-                        .HideHeader()));
-
-            // Add video games node
-            tree.AddNode("[red]Played video games[/]",
-                node => node.AddNode(
-                    new Table()
-                        .RoundedBorder()
-                        .AddColumn("Title")
-                        .AddColumn("Console")
-                        .AddRow("The Witcher 3", "XBox One X")
-                        .AddRow("Cyberpunk 2077", "PC")
-                        .AddRow("Animal Crossing", "Nintendo Switch")));
-
-
-            // Add the fruit nodes
-            tree.AddNode("[green]Fruits[/]", fruits =>
-                fruits.AddNode("Eaten",
-                    node => node.AddNode(
-                        new BarChart().Width(40)
-                            .AddItem("Apple", 12, Color.Red)
-                            .AddItem("Kiwi", 3, Color.Green)
-                            .AddItem("Banana", 21, Color.Yellow))));
-
             AnsiConsole.WriteLine();
-            AnsiConsole.MarkupLine("[yellow]Monthly summary[/]");
+
+            // Render the tree
+            var tree = BuildTree();
             AnsiConsole.Render(tree);
         }
+
+        private static Tree BuildTree()
+        {
+            // Create the tree
+            var tree = new Tree("Root")
+                .Style(Style.Parse("red"))
+                .Guide(TreeGuide.BoldLine);
+
+            // Add some nodes
+            var foo = tree.AddNode("[yellow]Foo[/]");
+            var table = foo.AddNode(new Table()
+                .RoundedBorder()
+                .AddColumn("First")
+                .AddColumn("Second")
+                .AddRow("1", "2")
+                .AddRow("3", "4")
+                .AddRow("5", "6"));
+
+            table.AddNode("[blue]Baz[/]");
+            foo.AddNode("Qux");
+
+            var bar = tree.AddNode("[yellow]Bar[/]");
+            bar.AddNode(new Calendar(2020, 12)
+                .AddCalendarEvent(2020, 12, 12)
+                .HideHeader());
+
+            // Return the tree
+            return tree;
+        }
     }
 }