[FIX] *: transfer the dataset when changing background options target

*: web_editor, website

Steps to reproduce the bug:
- Add a Cover snippet on the website.
- Put a "Blur" filter on the background image.
- Save.
- Change the parallax from "Fixed" to "None".
- Save and edit.
=> The "Filter" option displays "None" but should display "Blur".

When changing the parallax, `setTarget()` is called. The goal of this
function is to transfer the `background-image` from the old target to
the new one. The commit modifies this function by adding the transfer of
the dataset information relative to the background image from the old
target to the new one. It also transfers the `o_modified_image_to_save`
class from the old target to the new one if needed.

task-3287330

Part-of: odoo/odoo#119596