Add the "Entity view mode" module to core

[jenlampton]

This simple module provides a lightweight UI for adding additional view modes, and provides expected template suggestion hooks. https://www.drupal.org/project/entity_view_mode

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PR by @docwilmot: backdrop/backdrop#1645 initial port
Latest PR by @jenlampton backdrop/backdrop#1689

Yes please! 👍

[quicksketch]

Like other "Add X module to core" issues, I'd like to clarify that we're not planning on just adding a port of the module directly into core. The functionality from this module should be integrated directly into either the core entity.module or field.module.

[olafgrabienski]

In my opinion, custom view modes (or display modes) are especially helpful when you work with references. (Quick example: two content types, let's say Team member and Event. On the Event page you want to reference Team members who registered for the Event, but not the full information but only name and photo)

As Reference(s) is/are going to be integrated in core in 1.6, the need to build custom display modes via the UI will increase, so let's add the ability to do so in release 1.6 as well. @quicksketch @jenlampton What do you think about it?

See also @quicksketch's statement on #2084 (comment):

One feature from DS that could (and should) be implemented as soon as possible is the ability to create custom display modes.

[klonos]

This is part of #378, as in D8 they have already implemented this feature...

From #2250 (closed as duplicate of this issue):

In D6 there was Build Modes and then its successor Entity View Modes for D7 that allow you to create custom view modes through the UI. It seems that in D8 they have implemented something like that in core (or a lite version of the respective feature of Display Suite)...

They have added a new "Display modes" menu under /admin/structure that has two submenus: "Form modes" and "View modes":

"Form modes" I guess allows to manage the fields in the "Manage form display" tab (which we don't have in Backdrop - separate issue), while "View modes" allows you to manage the existing and add new view modes.

[mikemccaffrey]

It has always struck me as weird that you can't arbitrarily create new view modes in the ui, since there is so little to them.

The only tricky part is figuring out how to handle it I think the ui. Adding an interface under structure seems like a good idea. And when you create one, you can just enter the name and then check which entity types it should be available for.

[olafgrabienski]

Adding an interface under structure seems like a good idea. And when you create one, you can just enter the name and then check which entity types it should be available for.

Additionally, we could add a link "Add new custom display" on the Manage Display page of a content type, that would be under the "Custom display settings".

Side note: "Custom display (setting)" seems to be the Backdrop name for view mode, so Structure > Custom displays?