create separated widget mod implementation

src/browser/header/widget.rs

@@ -0,0 +1,18 @@
+pub struct Header {
+    gtk: gtk::HeaderBar,
+}
+
+impl Header {
+    pub fn new(tray: &gtk::Box, subject: &gtk::Box) -> Header {
+        let gtk = gtk::HeaderBar::builder().build();
+
+        gtk.pack_start(tray);
+        gtk.set_title_widget(Some(subject));
+
+        Self { gtk }
+    }
+
+    pub fn gtk(&self) -> &gtk::HeaderBar {
+        &self.gtk
+    }
+}