Preserve entangled fields deepcopy with nested lists and tuples for have vertical or horizontal fieldsets capability

[haricot]

Hoping it is a right approach !
For proof of concept, I will do a PR about this with Djangocms-cascade.

[haricot]

if not more simply by changing the template_name attribute of each widget as in the cascade plugin
https://github.com/jrief/djangocms-cascade/blob/29e7357cccec5a0e78f1498a95aef9ca7ce53e96/cmsplugin_cascade/bootstrap4/container.py#L31
it would be the same result, with more customization possible.
I think close this PR, which is not necessarily necessary suddenly.

[haricot]

After reflextion this is very convenient instead of overloaded all the template_name attributes needed for some widgets, it follows the logic of the django fieldsets.
With a little logic it allows to have 3 choices possibllity different custom fieldsets to choose from:

['glossary', 'somefields', 'somefields']

['glossary', ['somefields', 'somefields']]
 
(None, {
   'classes': ('custom_help',),
    'fields': (
        ('some_fields'),('some_fields1'),  ('some_fields2')
    ),  
}),

So I am rather for.
I think that it avoids multiplying the number of template widget varitions or another logic custom.