Provide a `menu_button` alternative that creates a small button

It seems that `Ui::menu_button()` creates a normal `Button`, then augments it with the menu behavior. However, I have not found any way to access this button afterwards to modify it, such as calling `Button::small()` on it. Even superficially copying the code and making the changes to it wouldn't work because `BarState` is private.

Is there a way to access the button directly that I'm just missing? If not, please either:
1) Provide access to the button via some "getter"; or
2) Create a `Ui::small_menu_button()` method that calls `Button::small()` internally.

Start at Ui::menu_button and inspect how it creates and configures the Button, then compare the existing Button::small behavior. The issue proposes either exposing that button for further modification or adding Ui::small_menu_button; done means one of those alternatives is available without requiring access to private BarState.

Written by the indexing model from the issue text.