[FIX] web_editor: reorder the elements of the invisible elements panel

Steps to reproduce the bug:
- Add a form snippet on the footer of the website page.
- Change the visibility of the form snippet to "conditionally".
- On the footer settings, deactivate the "Page Visibility".
=> The form snippet is still present on the "Invisible Elements" panel
but clicking on it has no effects. Indeed, it is inside the footer which
is hidden.

The goal of this commit is to reorganize the "Invisible Elements" panel
in order to better visualize the hierarchy between the different
invisible elements. To do so, the list of the invisible snippet elements
(`[...$invisibleSnippets]`) is scanned. Each invisible snippet that is
the descendant of another is discarded of the list (to only keep the
"root" ones) and a map is created with its `keys` set to invisible
snippets that have invisible descendants. The `value` corresponding to
an invisible snippet element is a list filled with all its descendant
invisible snippets except those that have a closer invisible snippet
ancestor. The list of the root snippets is then scanned. Each root
snippet is inserted in the DOM as well as their descendant snippets
found thanks to the newly created map.

Note that thanks to this commit, another problem is solved:
- Add a cookie bar on the website.
- Add a Text-Image snippet and change its visibility to "conditionally".
- Save and edit again. Note that the "Cookies Bar" is above the "Text-
Image" on the "Invisible Elements" panel.
- Click to display the cookie bar.
=> The order of the "Cookies Bar" and the "Text-Image" is switched on
the "Invisible Elements" panel.
Indeed, before this commit, the order of the snippets was influenced by
the order of their snippet editor creation. Because this order was not
always the same from one call to `_updateInvisibleDOM()` to another, a
glitch could happen when toggling a snippet visibility.
This is now fixed as the order of the invisible snippets in the
"Invisible Elements" panel is determined either by their order in the
list `rootInvisibleSnippetEls` or their order in the lists contained as
value in the map `descendantPerSnippet`.

task-3203914

Part-of: odoo/odoo#113549