Edit the images of the gallery macro in place in the WYSIWYG editor

The content of the gallery macro is a plain text of image references in wiki syntax. The WYSIWYG editor shows the images, but they cannot be added, removed or reordered in place: changing the gallery needs the Edit Macro dialog, where adding an image means typing its reference, like image:Space.Page@alice.png.

The fix is done in GalleryMacro.java:

• Declare the content as rich text, so that the editor edits the images in place: add one with the image picker of XWIKI-23735, remove one, or drag it to another position.
• Keep a content area to type in when the gallery is empty, so that it can be inserted without the Edit Macro dialog and the cursor put there, see XWIKI-25563 and XWIKI-25569.

Depends on

• XWIKI-23735, which provides the image picker to add the images.

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 and XWIKI-25569, so that the gallery can be inserted without the Edit Macro dialog and the cursor put in it.