lupdate: Allow trailing commas in QtScript/JS object literals

As per ECMA-262 5th edition, section 11.1.5, trailing commas should
be allowed in object literals. E.g.

{ foo: 10, }

shouldn't cause a syntax error; the comma should simply be ignored.

Task-number: QTBUG-22970
Change-Id: I3e730ad3001760475e5e40bb4a28a05b0baf5c7e
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>