Ubo-gui

- fix: pass duration with value of `0` to `Screen`'s transition runner to avoid flickering of the screen

- build: update headless-kivy-pi to the latest version

- build: update headless-kivy-pi to the latest version

- fix(MenuWidget): the menu doesn't override the top-most item whenever an arbitrary sub-menu's subscription reports a change, instead, it updates the item in the stack for which the change was reported
- refactor(PromptWidget): `PromptWidget` now exposes an `items` list property to be compatible with other subclasses of `PageWidget` so that tests can interact with it easier

- fix(MenuWidget): render placeholder when the menu is empty and `render_surroundings` is `True`

- feat(MenuWidget): add methods for directly selecting an `Item`