This commit enables the use of the option "All pages" when using a popup
snippet.

Steps to reproduce the bug:
- Drag & drop a popup snippet.
- On its option in the right panel, there is "Show On" set by default
  to "This page".
- Try to modify this option and select "All pages".

Bug observed: The "All pages" option can not be selected and the "Show
On" stays on "This page".

This commit has been done because since the version 16.0, the footer is
on an iframe [1]. Because of that, the program does not have a direct
access to the footer. This is why it is needed to first access the
top-level document before using the querySelector.

[1]: https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b



task-2983822

closes odoo/odoo#103641

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