Field group functionality in core

Update:

The current plan for fieldgroup in core is to revamp the fieldgroup API, and add groups for all the types of groupings that already exist in form API. The new fieldgroup API will become part of core's field + field_ui modules.

Group types planed for core: * Fieldset * Details * Div * Vertical Tabs

Group types that should remain in contrib: - HTML element - HTML5 - Horizontal tabs - Multipage - Accordion

The Field Group contrib module would simultaneously release a new minor version (that depends on this version of Backdrop core) where those group types are removed from the module, and all remaining types are updated to use the core field group API (which will be similar to what's there already). The current group API will also be removed from the contrib module.

Requirements for getting new fieldgroup contrib module in core: - [ ] Integrate Fieldgroup API into field/field_ui modules - [ ] Create group type for fieldset - [ ] Create group type for details - [ ] Create group type for div - [ ] Create group type for vertical tabs

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Original Issue: It would be pretty outstanding if we have field_group in core so that forms could be arranged a lot nicer with things like vertical tabs and such through the UI.

Requirements for getting fieldgroup contrib module in core: - [x] Place the module into the core/modules directory - [x] Move theme functions into field_group.theme.inc file - [x] Remove all the field group help text and fieldset - [x] Field Group Configure and Delete operations should use dropbutton - [x] Field Group Configure form should use menu callback (move from an expanding section to a new page -- same as field settings). - [x] Remove the t() function from all tests - [x] Move display tests into field_group.display.test - [x] Standardize on Field group for the UI and documentation (as opposed to Fieldgroup in some places). - [x] Adjust very long lines to be under the 80 character limit for coding standards - [x] Move display suite code to ds module: see https://github.com/backdrop-contrib/ds/issues/11
- [ ] Add description text for the div settings (what are "effect", and "speed"?) - [ ] Combine the options for html_element and html5 and provide an upgrade path - [ ] Replace the new stdClass(); in _field_group_get_html_classes().

User experience cleanup options: - [x] N/A ~Remove the whole 'Fieldgroups' fieldset and help text~ (from previous iteration of module) - [x] Remove the whole 'Fieldgroups' vertical tab (including the ability to clone from another display)

New features (options): - Make field group containers collapsible - Add a "clone" operation for a field group on the currently visible display - Instead of having a separate 'Add new group' row below 'Add new field', just have 'Group' as an option under 'Select a field type', and the different group types (e.g. fieldset, vertical tab, etc.) as 'Widgets'.

- (is this a good idea?) remove the ability to change the group type from the overview page (why?)

Related issues: * Switch to an "Add field" and "Add field group" action link (same as D8/D9): * Separate "Manage fields" and "Manage form" (field group would not appear on "Manage Fields")

Issue advocate: @jenlampton