[16.0] web_group_expand: show buttons in account ListView

[robperezit]

The account module defines a primary ListView buttons template (account.ListView.Buttons) used in invoices/bills lists (e.g. to add the Upload button). Since web_group_expand only extended web.ListView.Buttons, its expand/collapse buttons were not rendered in those views.

Extend account.ListView.Buttons too, injecting the buttons in o_list_buttons when the list is grouped.

#3123

[CRogos]

I think this would require a dependency on the account module?

I think a new module account_group_expand would be best.

[robperezit]

Thanks for the review! @CRogos

To align with OCA conventions: would you prefer

1. adding a small glue addon in this repository/branch (e.g. web_group_expand_account)
   depending on web_group_expand + account, containing only the
   account.ListView.Buttons template extension, or
2. keeping this repository strictly generic and moving the account-specific part
   to another repo/addon instead?

I can adapt the PR accordingly (split into two PRs if needed).

[CRogos]

You are asking the right questions... I don't know.
https://oca.github.io/repo-maintainer-conf/teams.html#web-maintainers

@simahawk @gurneyalex ?