move new tab button into the tabs start widget

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

@@ -0,0 +1,35 @@
+use gtk::{
+    gio::SimpleAction,
+    prelude::{ActionExt, ButtonExt},
+    Align, Button,
+};
+use std::sync::Arc;
+
+pub struct Widget {
+    gobject: Button,
+}
+
+impl Widget {
+    // Construct
+    pub fn new_arc(action_tab_append: SimpleAction) -> Arc<Self> {
+        // Init gobject
+        let gobject = Button::builder()
+            .icon_name("tab-new-symbolic")
+            .css_classes(["flat"])
+            .valign(Align::Center)
+            .tooltip_text("New tab")
+            .build();
+
+        // Init events
+        gobject.connect_clicked(move |_| {
+            action_tab_append.activate(None);
+        });
+
+        Arc::new(Self { gobject })
+    }
+
+    // Getters
+    pub fn gobject(&self) -> &Button {
+        &self.gobject
+    }
+}