[FIX] web_editor: do not show overlay for invisible elements

When a snippet is out of the viewport or hidden, the overlay should
not be visible. Two distinct cases we had:

- When the page is scrolled, the header is sometimes hidden thanks to
  tranform: translate(0, -100%); for some kind of effect -> the header
  is thus still visible but out of the viewport, any selected snippet
  inside that header would still have this overlay enabled so we would
  see the move and delete button on the top of the screen.

- When the page is scrolled, with the vertical header, some part of the
  header are hidden for the affixed version. If any snippet in those
  hidden elements was selected, the overlay is still visible floating.

task-2312878

closes odoo/odoo#58383

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Co-authored-by: Benjamin Vray <bvr@odoo.com>
Co-authored-by: Thanh Dodeur <tso@odoo.com>