gtk: append new tabs at the end if config is set

This adds a new config option: `window-append-new-tabs` (please: if you
have a better name, let me know). If this is set to true, then new GTK
tabs aren't added after the current tab, but after at the end.

src/apprt/gtk/Tab.zig

@@ -107,13 +107,21 @@ pub fn init(self: *Tab, window: *Window, parent_: ?*CoreSurface) !void {
     const gl_area_widget = @as(*c.GtkWidget, @ptrCast(surface.gl_area));
     c.gtk_box_append(self.box, gl_area_widget);
 
-    // Add the notebook page (create tab). We create the tab after our
-    // current selected tab if we have one.
+    // Add the notebook page (create tab).
+    const parent_page_idx = parent_page_idx: {
+        // If this option is enabled we append the new tab to the end of the notebook.
+        if (window.app.config.@"window-append-new-tabs") {
+            break :parent_page_idx c.gtk_notebook_get_n_pages(window.notebook);
+        }
+        // Otherwise we add it after the current tab.
+        break :parent_page_idx c.gtk_notebook_get_current_page(window.notebook) + 1;
+    };
+
     const page_idx = c.gtk_notebook_insert_page(
         window.notebook,
         box_widget,
         label_box_widget,
-        c.gtk_notebook_get_current_page(window.notebook) + 1,
+        parent_page_idx,
     );
     if (page_idx < 0) {
         log.warn("failed to add page to notebook", .{});

src/config/Config.zig

@@ -656,6 +656,10 @@ keybind: Keybinds = .{},
 /// Currently only supported on macOS.
 @"window-step-resize": bool = false,
 
+/// Append new tabs to the end of the tab list instead of right after the
+/// current tab.
+@"window-append-new-tabs": bool = false,
+
 /// When enabled, the full GTK titlebar is displayed instead of your window
 /// manager's simple titlebar. The behavior of this option will vary with your
 /// window manager.