Before this commit, there were cases where toggling the grid mode did
not work correctly. These cases are:
- when a snippet has None columns (e.g. Cover) and that an inner snippet
containing a row (e.g. Form) was dropped into it,
- when a snippet has some content outside its row (e.g. Picture) and
that an inner snippet containing a row was dropped in this outer
content,
- when a snippet has some content outside its row (e.g. Picture) and
that an inner content (e.g. Alert) was dropped in this outer content.

The first two cases happen because the row that was taken into account
when toggling the grid mode was the first `.row` element found in
general, instead of only considering the container children. Therefore,
what happened in these problematic cases is that the row found was the
inner snippet one, which should not be the case and caused the toggle to
fail because it was trying to put the element inside itself.

The third case happens because the `div` elements were excluded when
placing the outer content in a column, instead of only excluding the row
element.

This commit fixes these issues by only considering the container
children when looking for the row element when toggling the grid mode,
and by filtering out only the row element instead of every `div` when
creating a column for the outer content.

task-3279191

closes odoo/odoo#119965

Signed-off-by: Dieleman Guillaume <gdi@odoo.com>