Describe the bug
When using Ui::menu inside of Ui::menu, although it is possible to enter the top-level menu using the Enter or Space key, there is apparently no (intuitive) way to enter a submenu inside of that menu via keyboard, it is only able to enter it via mouse, which is obviously bad from an accessibility perspective.

To Reproduce
Steps to reproduce the behavior:

1. use the following in an appropriate location to setup a menu bar.

let mut tmp = false;
egui::TopBottomPanel::top("menu_bar")
    .show(ctx, |ui| {
        egui::menu::bar(ui, |ui| {
            ui.menu_button("X", |ui| {
                ui.menu_button("Y", |ui| {
                    ui.checkbox(&mut tmp, "Goal");
                });
            });
        });
    });

2. Navigate to the menu button "X" using keyboard, open it via Enter
3. Navigate to the submenu button "Y" using keyboard, in the now-open menu, observe that it can't be opened via Enter.

Expected behavior
The submenu should open when Enter is pressed on the keyboard.

Desktop (please complete the following information):

• OS: Linux
• Version: various; egui: 0.29.1 and master

Additional context
See also downstream bug: https://codeberg.org/topola/topola/issues/36 and demo/screencapture https://codeberg.org/attachments/4c923170-f824-4b7d-926c-54ac0622c395