Add a SegmentedButton widget

### What is the problem or limitation you are having?

Many platforms provide a widget that presents as a single horizontal button, where different segments can be selected - for example:

Toga does not currently have a widget for this type of construct.

### Describe the solution you'd like

Toga should gain a new SegmentedButton widget. The API for this button will be almost identical to Selection, except that it will also have a `multiple_selection` option (with semantics matching Table's handling of selections).

Suggested implementations:

* Android: [Segmented button](https://m3.material.io/m3/pages/segmented-buttons/overview) (or a [button group](https://m3.material.io/components/button-groups/overview) - Button Group is the current suggested approach, but was only introduced in May 2025; availability may be a concern)
* GTK3: [GTKButtonBox](https://docs.gtk.org/gtk3/class.ButtonBox.html)
* GTK4: [https://docs.gtk.org/gtk4/class.ToggleButton.html]
* Qt: [QButtonGroup](https://doc.qt.io/qtforpython-6/PySide6/QtWidgets/QButtonGroup.html)
* macOS: [NSSegmentedControl](https://developer.apple.com/documentation/appkit/nssegmentedcontrol?language=objc)
* iOS: [UISegmentedControl](https://developer.apple.com/documentation/uikit/uisegmentedcontrol?language=objc)

GTK4 ToggleButton doesn't appear to support multiple selection - when this occurs, the platform should warn the user on construction, but return the *semantics* of multiple selection (i.e., selection is a list) within a single-selection implementation.

### Describe alternatives you've considered

Don't support this type of widget. However, given the prevalence of this type of widget in practice, that seems undesirable.

### Additional context

This is functionally equivalent to a Radio Button, but the rendering is easier because it's guaranteed to be on a single line (rather than requiring a grouping syntax).

There may be an opportunity to have a common base class implementation for SegmentedButton and Selection - any opportunity to provide a common base class should be taken.

There will be be overlap on iOS in the updated implementation of OptionContainer (See #4297) - a SegmentedControl is the basis for building a container based on that selection.

Start by reading Toga's existing Selection and Table APIs, then inspect their platform implementations and the related OptionContainer work in #4297. Compare the suggested Android, GTK, Qt, macOS, and iOS controls and determine how their selection behavior maps to the proposed API. Done means a consistent SegmentedButton API and supported platform implementations with tests for single and multiple selection semantics.

Written by the indexing model from the issue text.