implement separated mod for window widget, rename getters

src/app/browser/window/widget.rs

@@ -0,0 +1,20 @@
+use gtk::{prelude::BoxExt, Box, Notebook, Orientation};
+
+pub struct Widget {
+    gobject: Box,
+}
+
+impl Widget {
+    // Construct
+    pub fn new(tab: &Notebook) -> Self {
+        let gobject = Box::builder().orientation(Orientation::Vertical).build();
+        gobject.append(tab);
+
+        Self { gobject }
+    }
+
+    // Getters
+    pub fn gobject(&self) -> &Box {
+        &self.gobject
+    }
+}