[D8] Manage form display: Allow managing form fields non-programmatically

Drupal 8+ has separate "Manage display" and "Manage form display" tabs:

• The "Manage display" UI allows you to reorder/configure the various fields when they are being rendered/viewed.
• The "Manage form display" UI allows you to reorder/configure the various fields as they are shown during data entry.

D7 and Backdrop only have the "Manage display" UI, but in D7 one could use the Display Suite contrib module to manage the display of fields in forms (the "Display Suite Forms" submodule to be exact: https://www.drupal.org/node/1795380). The Display Suite module has not been ported to Backdrop though, since most of its features have been implemented in Backdrop in the form of the core Layout module (see https://docs.backdropcms.org/documentation/features-added-to-core).

We should consider adding this functionality in core, to reduce resistance when deciding to choose Backdrop over Drupal 8+.