create separated widget mod implementation

src/browser/main/widget.rs

@@ -0,0 +1,23 @@
+use gtk::prelude::BoxExt;
+
+pub struct Main {
+    gtk: gtk::Box,
+}
+
+impl Main {
+    // Construct
+    pub fn new(tab: &gtk::Notebook) -> Main {
+        let gtk = gtk::Box::builder()
+            .orientation(gtk::Orientation::Vertical)
+            .build();
+
+        gtk.append(tab);
+
+        Self { gtk }
+    }
+
+    // Getters
+    pub fn gtk(&self) -> &gtk::Box {
+        &self.gtk
+    }
+}