Old thumbnails for deprecated snippets have been removed. The
other snippet thumbnails have been grouped together in same folder.
Some images are unnecessary since the new mail builder, some are simply
not used anywhere.
The references snippet is limited to four images, adding more columns
will copy the last column's image, so no need for images 5 and 6.
The basic template doesn't allow snippets - the editor doesn't even
appear - since no snippets can be added, their images are unnecessary in
the templates image folder.
In all cases, the drag&dropped snippets, even from other templates,
use the theme_default images and not the templates' images.
Move all images in snippets_demo folder to theme_default folder
For more consistency, default images have been renamed according to
their snippet names
Part-of: odoo/odoo#95940
We shouldn't add a data-snippet in the declaration of a snippet,
as they are meant to be drag & dropped, the editor/right panel is going
to add that for you. We generally add them when we copy paste a snippet
definition into another view directly in our sources (ie: template)
Ref: https://github.com/odoo/odoo/pull/87059#discussion_r866903584
Part-of: odoo/odoo#95940
Elisabeth Dickinson