Wt

There have been some requests for improvements to TinyMCE (#11249 and #11267), so I'm creating this overarching issue to take a closer look at WTextEdit to see if it can be modernized a bit.

This could include:

• Verifying that WTextEdit works properly with all versions of TinyMCE >= 3? We know that 3 and 4 work, but I'm not 100% sure about the state of 5 and 6 support.
• Bundle TinyMCE 6 (MIT License) with Windows releases and JWt? JWt currently bundles 3 and 4, and Windows releases include TinyMCE 3.
• Breaking change: change the default TinyMCE version to 6? Alternatively, can we detect which version is being used?
• Have the deployed widget gallery (https://www.webtoolkit.eu/widgets/forms/line_text-editor and http://jwt.emweb.be/jwt-gallery/gallery/forms/line_text-editor) use TinyMCE 6
• Issue #11249: Support Wt::WTextEdit::setPlaceholderText
• Issue #11267: Support changing readonly state after editor is rendered.