Allow editing partially shrunk columns
* lisp/org-table.el (org-table-with-shrunk-field): New macro.
(org-table-get-field):
(org-table-toggle-column-width): Use new macro.
(org-table--shrunk-field): Update function.
(org-table--shrink-field): When there is a width cookie, leave first
characters editable.
* lisp/org.el (org-self-insert-command):
(org-delete-backward-char):
(org-delete-char): Small refactoring. Handle shrink overlays.
* testing/lisp/test-org-table.el (test-org-table/toggle-column-width):
Update tests.