Wt

JavaScript layouts are a fully custom mechanism to render out, and position content.
They stem from before flex boxes were nicely supported by all browsers.

Flex is now a generally available, and supported mechanism (and has been for years).
We should discourage users from using the custom layout managers implementations, and default to Flex.

The current [WLayout::defaultImplementation()]](https://www.webtoolkit.eu/wt/doc/reference/html/classWt_1_1WLayout.html#adcefa1eeb199ab534e121100fc7fcfe1) is already Flex. But the JavaScript versions should be deprecated.

In the future they can be fully removed.