Edit the nodes of a static tree in place in the WYSIWYG editor

A static tree is described by the content of the tree macro, a nested list of nodes in wiki syntax. The macro declares this content as a plain string, so the WYSIWYG editor never edits it in place: the editor shows the list, which cannot be edited, and adding, renaming or moving a node needs the Edit Macro dialog, where the nodes are typed as a wiki syntax list. Right after the insertion from Quick Actions, the macro has no content area at all, so the text typed next goes outside the macro.

The fix is done in Tree.xml:

• Declare the content as rich text, so that the editor edits the nodes in place as a nested list, with the list tools of the editor to indent and outdent them.
• Keep a visible content area to type in when the tree is empty, which one click puts the cursor in, so that the cursor can also be put there after the insertion, see XWIKI-25569.

KOs covered

Impact found on macro fluidity

These 2 KOs come from the WYSIWYG usability audit of the macros, under the criteria E6 inplace and E5 empty of its usability criteria, which ask 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, and that an empty content stays visible and editable, with no area that cannot receive the cursor. The page of the macro, linked in the table, has the full notes and the steps to reproduce. It is also needed by XWIKI-25569, so that the cursor can be put in the content of the tree after the insertion.