use libadwaita for app & app window

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

@@ -0,0 +1,23 @@
+use adw::{HeaderBar, WindowTitle};
+use gtk::Box;
+
+pub struct Widget {
+    gobject: HeaderBar,
+}
+
+impl Widget {
+    // Construct
+    pub fn new(pack_start: &Box, title_widget: Option<&WindowTitle>) -> Self {
+        let gobject = HeaderBar::builder().build();
+
+        gobject.pack_start(pack_start);
+        gobject.set_title_widget(title_widget);
+
+        Self { gobject }
+    }
+
+    // Getters
+    pub fn gobject(&self) -> &HeaderBar {
+        &self.gobject
+    }
+}