library: add CenterBox entry

src/Library/demos/Center Box/main.blp

@@ -0,0 +1,46 @@
+using Gtk 4.0;
+using Adw 1;
+
+Adw.StatusPage {
+  title: _("Center Box");
+  description: _("Displays three child widgets, while keeping the middle centered");
+
+  child: Box {
+    spacing: 24;
+    orientation: vertical;
+
+    Adw.Clamp {
+    maximum-size: bind width_adjustment.value;
+    child: CenterBox {
+        shrink-center-last: true;
+
+        [start]
+        Switch {
+          valign: center;
+        }
+
+        [center]
+        Entry {}
+
+        [end]
+        CheckButton {}
+      };
+    }
+
+    Scale width_scale {
+      orientation: horizontal;
+      halign: center;
+      width-request: 120;
+      adjustment: Adjustment width_adjustment {
+        lower: 200;
+        upper: 500;
+        value: 325;
+      };
+    }
+
+    LinkButton {
+      label: "API Reference";
+      uri: "https://docs.gtk.org/gtk4/class.CenterBox.html";
+    }
+  };
+}

src/Library/demos/Center Box/main.json

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+{
+  "category": "layout",
+  "description": "Displays three child widgets, while keeping the middle centered",
+  "panels": ["ui", "preview"],
+  "autorun": true
+}