[D8] theme_table() should take an optional footer variable and produce tfoot

This is the same as https://www.drupal.org/project/drupal/issues/1892654 for D7, which is a backport of what has already been done for D8 in https://www.drupal.org/project/drupal/issues/806982 (which makes it part of #378)

Problem/Motivation

Somehow the <tfoot> element of a table never got into core. While not commonly used, it still is a necessary component on some sites. Themes have to add this functionality when its optional use should be provided by core.

Proposed resolution

Add optional #footer variable in theme hook and corresponding <tfoot> markup in table template.

PR: https://github.com/backdrop/backdrop/pull/2554