Support of block-based widget editor (WordPress 5.8)

[herrvigg]

The fix for the widget editor #1042 was quite difficult to achieve, but it seems to work fine. This is using the Classic TinyMCE editor.

With WordPress 5.8 though, they have just enabled the support for the block-based widget editor.
It is enabled by default... which makes the widget fix obsolete... 😓
Hopefully (!) I found a a solution thanks to the Classic Widgets plugin. This plugin is just adding two lines of code to disable it, that I already integrated in the widget fix (#1042).

The widget block editor is a new feature difficult to support in qTranslate-XT since it's a different mechanism (REST and JS code): https://github.com/WordPress/gutenberg/tree/trunk/packages/edit-widgets.
It may be possible to achieve an initial support that is single-language based as for the post. But it's very difficult to dig into the Gutenberg code... Again this will require new hacks.

So this is the situation. I plan to release the current version in the current state, meaning that qTranslate-XT enforces the classic editor for the widgets. But it's still a bit unclear if this should be done only for specific widgets, since qTranslate-XT only handles the text widget... it's getting complicated.