contrib/lisp/org-contacts: Allow org links in properties

contrib/lisp/org-contacts: Allow org links in properties

* contrib/lisp/org-contacts.el (org-contacts-split-property):
  Introduce a custom version of split-string that keeps org links
  intact.

* contrib/lisp/org-contacts.el (org-contacts-strip-link): Introduce a
  new function that removes brackets, description, link type and colon
  from an org link string returning the pure link target.

* contrib/lisp/org-contacts.el (provide 'org-contacts): Remove a
  redundant line.

* contrib/lisp/org-contacts.el (org-contacts-complete-group,
  org-contacts-complete-name, org-contacts-view-send-email,
  org-contacts-get-icon, org-contacts-vcard-format): Apply the new
  functions to the already existing functions extracting telephone
  numbers and email addresses from the properties.

Allowed separators for email addresses and phone numbers are `,', `;'
and whitespace.  `:' is not allowed anymore as separator to avoid
confusion with implicit links.

Examples of properties that are possible after those changes:

* Surname, Name
:PROPERTIES:
:EMAIL:    mailto:test2@test.de; [[mailto:name@test.de]] foo@bar.biz
:PHONE:    [[tel:+49 351 4129535]], +491766626196 [[+49 (351) 41295-35]]
:END:

Phone links of the form [[tel:+49 351 412 95-35][My phone number]] or
[[tel:+49 351 41295-35]] are expected.  `-', `/', `(', `)' and
whitespace characters are allowed in telephone numbers.