Add custom layout creation.

src/Layout.zig

@@ -157,6 +157,22 @@ pub fn remove(self: *Layout, node: *Node) void {
     self.alloc.destroy(parent);
 }
 
+/// Build a split holding two existing subtrees.
+///
+/// `insert` grows a tree one pane at a time, which is the right shape for
+/// interactive splitting but can't express an arbitrary arrangement with
+/// per-split ratios. Opening a saved layout needs exactly that, so it builds
+/// the tree bottom-up through this instead.
+pub fn newSplit(
+    self: *Layout,
+    orientation: gtk.Orientation,
+    a: *Node,
+    b: *Node,
+    ratio: f64,
+) !*Node {
+    return self.makeSplit(orientation, a, b, ratio);
+}
+
 fn makeSplit(
     self: *Layout,
     orientation: gtk.Orientation,