Cocoa: Handle insertNewLine selector for when it is sent while composing text

Some IMEs will perform an insertNewLine when Return/Enter is pressed while
composing text as well as causing the edit to finish. By handling this it
will ensure that the extra enter/return event is sent when the IME
requests it.

Task-id: QTBUG-39125
Change-Id: Ice6eded68e6b1dc51703a38316f76f78099923da
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>