Editor mixin creation

Following the identification of bugs in the solid-form-richtext widget, an alternative mixin using the WYSIWYG editor tinyMCE has been investigated.

An editor mixin has been created (see MR !661 (closed) ) however two points must be considered :

• there is a problem with importing the CSS file, which produces a console error (does not block the use of the mixin): must test the import via CDN on editorMixin.ts page
• several tests are problematic, Cypress is demanding about iframe tests : testing two solid-form-editor on the same page raises errors and retrieving field values from the data-src fails. However, the demo examples (widget-richtext-markdown-editor.html) illustrate that the expected behavior is effective.