gtk: don't allow focusing on tab overview or menu buttons

This prevents GTK from focusing on those buttons, which would prevent
focus from returning to the terminal after those buttons had been used.

src/apprt/gtk/Window.zig

@@ -214,6 +214,7 @@ pub fn init(self: *Window, app: *App) !void {
     {
         const btn = gtk.MenuButton.new();
         btn.as(gtk.Widget).setTooltipText(i18n._("Main Menu"));
+        btn.as(gtk.Widget).setCanFocus(0);
         btn.setIconName("open-menu-symbolic");
         btn.setPopover(self.titlebar_menu.asWidget());
         _ = gobject.Object.signals.notify.connect(
@@ -253,6 +254,7 @@ pub fn init(self: *Window, app: *App) !void {
             },
         };
 
+        btn.setCanFocus(0);
         btn.setFocusOnClick(0);
         self.headerbar.packEnd(btn);
     }