Declare the content of the footnote macro as rich text

The content of the footnote macro is wiki syntax, rendered with its formatting, but the macro declares it as a plain string. The WYSIWYG editor therefore never edits it in place: changing a footnote needs opening the Edit Macro dialog, where it is a plain text area of wiki syntax.

The fix is done in FootnoteMacro.java:

• Declare the content as rich text, a list of blocks, so that the editor can edit it in place with the formatting toolbar.
• The footnote macro is inline, and the content of an inline macro cannot be edited in place yet, see XWIKI-19954.

Depends on

• XWIKI-19954, which adds the in-place editing of the content of inline macros.

KOs covered

Impact found on macro fluidity

This KO comes from the WYSIWYG usability audit of the macros, under the criterion E6 inplace of its usability criteria, which asks that the content is edited in place, with an editing experience suited to its data, not in a text area of the Edit Macro dialog. The page of the macro, linked in the table, has the full notes and the steps to reproduce. It is also needed by XWIKI-25563, so that the Quick Action can insert the macro without the Edit Macro dialog.