Improvements for the HTML sandbox feature:
- name the formfield consistently with its ancestry
- provide two preview and visualization styles: inline, where the page is displayed alongside the code and in an own window, opened by clicking on a button
- allow to customize whether one prefers the inline style preview, the window preview or none via the preview_inline and preview_window parameters
- fix visualization when formfield is rendered as readonly and no monaco editor is available
- localize button text
