`SubMenu` arrow icon does not render without a supporting font

**Describe the bug**
Sub menu items do not render their arrow icon if current fonts do not support the "⏵" glyph:

Looking at egui code for `SubMenu`, it is indeed using the "⏵" glyph.
```rust
impl SubMenu {
fn new(parent_state: Arc>, text: impl Into) -> Self {
let index = parent_state.write().next_entry_index();
Self {
button: SubMenuButton::new(text, "⏵", index),
parent_state,
}
}
}
```

**To Reproduce**

Steps to reproduce the behavior:
1. Replace default fonts with custom fonts
2. Add a nested `ui.menu_button(...)`
3. Observe "?" being displayed instead of "⏵" on the submenu item

**Expected behavior**
The submenu arrow icon should be font-agnostic, like `CollapsingHeader` arrows:


Start by inspecting the `SubMenu::new` implementation and its `SubMenuButton`, then compare the submenu arrow with the font-agnostic arrows used by `CollapsingHeader`. Reproduce the issue with custom fonts and a nested `ui.menu_button(...)`; done means the submenu arrow renders without requiring the "⏵" glyph.

Written by the indexing model from the issue text.