create separated mod for widget

src/app/browser/window/tab/widget.rs

@@ -0,0 +1,19 @@
+use gtk::Notebook;
+
+pub struct Widget {
+    gobject: Notebook,
+}
+
+impl Widget {
+    // Construct
+    pub fn new() -> Self {
+        let gobject = Notebook::builder().scrollable(true).build();
+
+        Self { gobject }
+    }
+
+    // Getters
+    pub fn gobject(&self) -> &Notebook {
+        &self.gobject
+    }
+}