Better support for HTML/CSS in QTextDocument

This is a meta task with some improvements to the rich text support in Qt to support better html/css. The goal is not to support a full web engine but mostly focusing on small improvements which could already improve considerably QoL when using QTextDocument

• max-width: 100% for image. Implemented in https://codereview.qt-project.org/c/qt/qtbase/+/554130 !
• Support <del> and <ins> HTML tag. The html output of a lot of markdown to html converter output this and <del> is basically an alias to <s> which Qt already support.
• Support border-radius in QTextFormat. Usecase: user mentions in a chat app, rounding of images, rounding of tables, ... QTBUG-98334
• border for blockquotes. Quite useful to render plain text emails or threads in general in communication apps
• Basic CSS class support QTBUG-75662