[FIX] web_editor: show dropzones at an over after an over event

Steps to reproduce the bug:
- Add a "Text-Image" snippet on the website page.
- Take an "Inner content" snippet from the right panel and drag it from
a dropzone to another. It has to be fast so that the order of the "over"
and "out" dropzone events is "over" > "over" > "out" and not "over" >
"out" > "over" as usual.

-> The exited dropzone is not correctly displayed.

The problem is that since [1], the website is inside of an iframe. Due
to that, the snippets menu does not have a direct access to the
dropzones. To solve the problem, the already defined `$dropZones`
variable is used instead of searching the dropzones inside the DOM. For
the snippet editor, the problematic line has been deleted at it is
already handled by `_outPreviousDropzone()`. This is acceptable since
the commit that introduced `_outPreviousDropzone()` ([2]) and the commit
that introduced the removed line ([3]) were both introduced to handle
the case of an "over" after an "over". In other words, as
`self.droppped` will always be `true` when
`self.dragState.currentDropzoneEl` is set and it will always be `false`
when `self.dragState.currentDropzoneEl` is not set, the problematic line
can be removed as it is already handled in `outPreviousDropzone()`.

[1]: https://github.com/odoo/odoo/commit/31cc10b
[2]: https://github.com/odoo/odoo/commit/c6886410603a16b6a3c5fe459d273dd883c110c7
[3]: https://github.com/odoo/odoo/commit/fed5854e32bcddf8b14b8311b2fe542eed990f77

task-3522552

closes odoo/odoo#137255

X-original-commit: 06fde42d3ba0e8be2128cda8e695c5def917f391
Signed-off-by: Benjamin Vray (bvr) <bvr@odoo.com>

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -1113,7 +1113,6 @@ var SnippetEditor = Widget.extend({
             over: function () {
                 if (self.dropped) {
                     self.$target.detach();
-                    $('.oe_drop_zone').removeClass('invisible');
                 }
 
                 // Prevent a column to be trapped in an upper grid dropzone at
@@ -3283,7 +3282,8 @@ var SnippetsMenu = Widget.extend({
                             if (dropped) {
                                 $toInsert.detach();
                                 $toInsert.addClass('oe_snippet_body');
-                                $('.oe_drop_zone').removeClass('invisible');
+                                [...$dropZones].forEach(dropzoneEl =>
+                                    dropzoneEl.classList.remove("invisible"));
                             }
                             dropped = true;
                             $(this).first().after($toInsert).addClass('invisible');