[FIX] web_editor, website: correctly remove the image gallery snippet

Steps to reproduce the bug:
- Add an Image Gallery (IG) snippet on the page.
- Add 2 new images in the IG.
- Click on the first image of the IG to load its data.
- Click on the trash button to remove the snippet.
- Bug => The snippet is not removed (an image is removed instead).

When a snippet is removed, the `removeSnippet` function is called. The
problem is that the `call_for_each_child_snippet` will never resolve.
Two mechanisms are of interest to understand why: the first one is the
`updateCurrentSnippetEditorOverlay` function. Its goal is to destroy a
snippet each time its target is not in the DOM anymore. The second
mechanism is specific to the IG snippet: when an image of this snippet
is destroyed, the `slideshow` function goes through the remaining
images to update parameters. To do it, the function uses the
`_replaceContent` function that empties the content of the carousel and
then fills it with new data.

When a snippet is removed, a `SnippetEditor` is created for each
element of it. In the case of the IG, a `SnippetEditor` is created for
each image of the the snippet. Because the first image already has a
`SnippetEditor` (because it has been clicked), the callback of
`call_for_each_child_snippet` is called to remove this image from the
IG snippet. The second mechanism explained before will then be called.
Meanwhile, a `SnippetEditor` will be created for the second image.
However, because the `_replaceContent` function emptied the content of
the carousel, the `updateCurrentSnippetEditorOverlay` function will
destroy the `SnippetEditor` of the second image as its target is not
considered present in the DOM anymore. Unfortunately, the
`call_for_each_child_snippet` still needed this `SnippetEditor` and
will never entirely resolve.

To solve this problem, the `removeSnippet` function is executed inside
a mutex. Because the mutex is also used by
`updateCurrentSnippetEditorOverlay`, we are sure that this function
will not destroy the snippetEditor while the `removeSnippet` is still
running.

task-3147271

closes odoo/odoo#111719

Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>