Implement LFUN_REFERENCE_TO_PARAGRAPH

Implement LFUN_REFERENCE_TO_PARAGRAPH

This function checks whether a paragraph (specified by ID) has a label.
If so, it simply inserts a reference to this at cursor position, if
not it inserts a label to that paragraph (pos 0) and then inserts
a reference at cursor position.

Needed to implement #1624 (insert cross references to items that do not
have yet a label)