[FIX] web_editor: fix dropzones with invisible element

Before this commit, dropzones were created around invisible elements
(e.g. the snippet popup). So we had 2 dropzones instead of one at some
locations.

After this commit, we avoid creating dropzones around elements that
must not have dropzones.

task-2312878

closes odoo/odoo#65198

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>