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.