[FIX] web_editor: prevents dropping a snippet in a non-editable area

Before this commit, it was possible to drop a snippet in a non-editable
area (e.g. dynamic snippets).

Steps to reproduce the bug:

- Drop a "Dynamic Products" snippet in a page.
- Drop a "Columns" snippet in the same page.
- Moves a column from the "Columns" snippet into the "Dynamic Products"
snippet thanks to the "drag and drop" button.
- Bugs => It works when it shouldn't.

task-3054763

closes odoo/odoo#113758

X-original-commit: 4d61c356
Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>