Provide a way for third-party widgets to register themselves

[freakboy3742]

What is the problem or limitation you are having?

Toga provides a list of built-in widgets; however, we hope to develop an ecosystem of third-party widgets.

Built-in widgets are able to from toga import SomeWidget, and the implementation of SomeWidget can call self.factory.SomeWidget() to instantiate a platform-specific implementation.

However, if a third-party widget requires a native implementation (e.g., a Cocoa wrapper, a Winforms wrapper etc), there's not currently any way to use that infrastructure to get a platform-specific implementation.

Describe the solution you'd like

It should be possible for a third-party package to define a widget that has platform-specific implementations.

Entry points should exist, both for defining the existence of a widget, and for registering platform-specific implementations of that widget. This way, installing a third-party widget package would make that widget available in the toga. namespace (or maybe in a toga.ext. extension namespace to prevent naming collisions), and third-party widgets could invoke self.factory.CustomWidget() to get the appropriate platform implementation.

In the best case, Toga would "eat its own dogfood", and use the same mechanisms for built-in widgets that third party widgets use.

Describe alternatives you've considered

If the entry-point approach is used, it may not be necessary to register the widgets, just the implementations. Requiring end users to say from toga_thirdparty import CustomWidget isn't unreasonable; the primary concern is that the implementation of CustomWidget needs to be able to invoke self.factory.CustomWidget() (or self.factory.toga_thirdparty.CustomWidget(), to ensure namespacing) to get a platform-specific implementation of the widget).

As an alternative to entry points, Toga's widget factory could include a mechanism for third-party widgets to register themselves on import. The end result is much the same; the only difference is where the registration process occurs.

Additional context

This has been a long-standing need, but came up recently in the context of #2927.

We're already using entry points to register backends; the interface that toga cocoa uses to register itself with toga core is accessible to third parties. Extending this to include widgets (or other platform-specific features) would make sense.

This should be accompanied by documentation that provides a "sample" third-party widget. This documentation should also encourage users to use the togax namespace, rather than the toga namespace for third-party widgets registered on PyPI.

[freakboy3742]

See also #2687 - import_backend() may be a natural place to have fallback logic for third-party backends.