Don't delete in-place when replacing a display property

Don't delete in-place when replacing a display property

When calling 'add-display-text-property' on a region of text that
already contains PROP, we first delete the old display specification
from the region.  If the region's 'display' property is a list of
display specifications, we need to avoid destructively modifying the
list; other regions of text could be using the same list object.  (For a
'display' property that's a vector or a single display spec, this
doesn't matter since we first make a new list in the code.)

In addition, be more careful when working with a display property like
((margin ...) ...).  This is a single display specification, not a list
of display specs.

* lisp/emacs-lisp/subr-x.el (add-display-text-property): Don't delete
in-place for list values.  Handle (margin ...) display specification
type correctly.

* test/lisp/emacs-lisp/subr-x-tests.el
(subr-x-test-add-display-text-property): Update test.

(cherry picked from commit 4a3c8e6e1df44b187b7286747e363232e8b4e0ea)