[FIX] web_editor, website: keep the text selection when changing columns

It was decided with [1] to keep the text selection, if there was one,
when changing a snippet's option.
It was not working for the layout_column option:
- Drop a text snippet,
- Click on the first paragraph (the text is selected),
- Change the number of columns,
=> The text selection is lost.

To add some columns on snippets like the s_text_block which has no rows
and columns, the layout_column option will first wrap its content into a
set of row and column, which will remove the selection.
To fix that, the selection is restored after wrapping the snippet's
content.

The same thing is done when removing all columns from a snippet (and
unwrapping its content).

Related to https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b

[1]: https://github.com/odoo/odoo/commit/e8112e2865ca449c4df9cd6147e9e24c4c6dcd41

task-2687506

Part-of: odoo/odoo#98352