Add localization support 1

[hydra]

Current state: work-in-progress.

TODO:

• API documentation.
• Additional testing.
• PR review and changes.

Initial cut of localization support using Fluent.

See examples/localized.rs.

Features:

• When translations are missing, the key is displayed.
• The first translation specified becomes the default when the system local doesn't match.
• Widgets can be wrapped in Localized to allow for multiple translations to be used at the same time. Use-case is a form designer or report viewer where the main application is in one-language, but you want to preview a form or report in a different language. It works in the same way to being able to override the theme for a widget.
• Supports dynamic translation arguments.

Screenshots:

In this screenshot, the window uses the default locale, and the first container shows a label using the window's default locale.
The second container shows a label using a specific locale. The third container is using uses a dynamic locale which can be changed using the radio buttons. It has buttons which update a dynamic value which is used as an argument for a translation.

In this screenshot, the language was changed to "Spanish (Spain)" and the "+" button was clicked once. Here we can see the translations were used.

Fluent translation file for 'es-ES' in the screenshots above is as follows:

message-hello-world = ¡Hola, mundo!
language-en-gb = Inglés (Gran Bretaña)
language-en-us = Inglés (Estados Unidos)
language-es-es = Español (España)
banana-counter-message =
    { $bananas_counter ->
        [one] Hay un plátano.
        *[other] Tienes { $bananas_counter } plátanos.
    }

In this screenshot, we can see what the UI looks like when no translations are provided.

Limitations:

• Translations are currently re-parsed for each opened window. Some translations cache could be used in the future.
• Window titles do not currently support localization.

Acknowledgements:

• Vizia (https://github.com/vizia/vizia) - For some Fluent API usage, thanks @geom3trik

[ecton]

These changes should hopefully fix the layout invalidation issues reported on Discord.

[ecton]

I've done a major refactoring on this to introduce the MaybeLocalized type and attempt to round out some of the localization support the way I've envisioned it working. I haven't gone through and updated the doc comments for everything yet, so some of the functionality may be slightly different than the documentation currently.

The example has been updated and that's probably the best place to see the diff of most of the API changes: 64d06a5#diff-42a98171141d7e7760d41d679d6f551d906f60bcf7ff44babd6c577c514c923f

[ecton]

I've finished these changes and am going to get this merged. If you have any feedback on my updates, please don't hesitate to either comment here, chat on Discord, or open up new issues! Thank you for getting this work started!