move new tab button into the tabs start widget

src/app/browser/window/header/bar/tab/append.rs

@@ -0,0 +1,24 @@
+mod widget;
+
+use widget::Widget;
+
+use gtk::{gio::SimpleAction, Button};
+use std::sync::Arc;
+
+pub struct Append {
+    pub widget: Arc<Widget>,
+}
+
+impl Append {
+    // Construct
+    pub fn new_arc(action_tab_append: SimpleAction) -> Arc<Self> {
+        Arc::new(Self {
+            widget: Widget::new_arc(action_tab_append),
+        })
+    }
+
+    // Getters
+    pub fn gobject(&self) -> &Button {
+        &self.widget.gobject()
+    }
+}