[FIX] web_editor: fix moving inner content in carousel slides

Before this commit, when moving inner content inside a carousel slide
(with the 'move' button), the carousel was jumping to the first slide.

Steps to reproduce the bug:

- In website edit mode, drag and drop a Carousel snippet onto the page.
- Move to the second slide.
- Click on the column to activate its overlay.
- Click and hold the "drag and drop" button of the column to start
dragging it.
- Release the mouse button.
- Bug: the carousel jumps to the first slide.

task-3006831

closes odoo/odoo#103086

Signed-off-by: Benoit Socias (bso) <bso@odoo.com>