[FIX] web_editor: correctly move snippets on a popup

Inside a scrollable popup, the scroll was not correctly triggered when
moving one of its snippets through drag and drop.

This commit correctly defines the SmoothScrollOnDrag.$scrollTarget when
a modal is shown.

task-2431469

closes odoo/odoo#78813

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