Prepare markers for insertions inside of a widget

Prepare markers for insertions inside of a widget

Recreating child widgets without recreating the parent widget
may lead to situations where the parent widget doesn't cover its
children or buttons entirely anymore.  This bug manifests as a
faulty fontification of children or buttons, for example.
(Bug#69941)

* lisp/wid-edit.el (widget--prepare-markers-for-inside-insertion)
(widget--prepare-markers-for-outside-insertion): New functions.
(widget-default-create): Use them.

* test/lisp/wid-edit-tests.el (widget-test-insertion-at-parent-markers)
(widget-test-insertion-at-parent-markers-2): New tests.

(cherry picked from commit 85113fcda97970bc2468f409278e27d6570fc76f)