[FIX] website_editor: use background image on the right element

Commit [1] addressed the following situation: when we have a parallax on
an element, it adds the background on a `<span>` child of said element
instead of applying it directly on the element. Retargeting the `<span>`
is needed to display all the options linked to the background image.

Following [1], building blocks whose children are allowed to have a
background image are also applied their 1st child's background when one
of their children's background is modified. This happens because, when
changing the background of a child item, `BackgroundOptimize` is called
both on the child and on the main snippet due to
`_onBackgroundChanged()` bubbling up and `_loadImageInfo()` targetting
any child in the tree with the class `.oe_img_bg`.

This commit prevents `_onBackgroundChanged()` from bubbling up and
modifies the behavior from [1] by making sure the element identified as
the target within `_loadImageInfo()` can only be either the original
target or the parallax element.

Steps to reproduce:
- Drop a carousel building block
- Change its style to bordered
- Change the background image of the 2nd slide
=> the background of the entire snippet is also changed. It is then
impossible to remove the image on the snippet.

[1]: https://github.com/odoo/odoo/commit/31ba906



task-3349669

closes odoo/odoo#124083

Signed-off-by: Outagant Mehdi (mou) <mou@odoo.com>