[FIX] web_editor: modify background element at option change

For background images, modifying options such as "Filter" does not
directly lead to the modification of their corresponding data attributes
in the DOM as those modifications are done at the save request. This
should not be the case. The goal of this commit is to, as for regular
images, modify their corresponding DOM data attributes at the option
update.

Steps to reproduce the bug:
- Add a Cover snippet on the website.
- Replace the background image by one of your own.
- Add a filter to the background image.
- Inspect the Cover snippet.
=> The `data-gl-filter` attribute has not been added in the DOM and you
have to click on "Save" for it to be actually added.

task-3287330

Part-of: odoo/odoo#119596