Implement new entity family "\...."

Implement new entity family "\...."

* lisp/org-element.el (org-element--set-regexps):
(org-element-entity-parser): Update regexps for entities.

* lisp/org-entities.el (org-entities): Add new entities.

* lisp/org.el (org-fontify-entities): Add comment.

* etc/ORG-NEWS (title): Introduce new entities.

"\..." are used to insert an arbitrary number of spaces in various
back-ends.  In particular, this family can be used to introduce
leading spaces within table cells.

Also, unlike to other entities, these can be used mid-word, e.g.,

  wo\....rd

Curly brackets can still be added before a dot, e.g.,

  This is the end\...{}.