#562 Link existing child

[Jeffrey S Smith]

Several places where there is an 'add child' option, a 'link child' option would be useful as well:
"Add a child to this family" -> "Link a child to this family"
"Add a son or daughter" -> "Link a son or daughter"
"Add a brother or sister" -> "Link a brother or sister"
"Add a child to create a one-parent family" -> "Link a child to create a one-parent family"

Note: I added the latter strings to languages/editor.en.php. I'm afraid I don't know anything
about the translation process with this project.

[Jeffrey S Smith]

Adds 'Link a child' functionality