Handle enum values of -1 correctly.

Handle enum values of -1 correctly.

This was already handled correctly most places; now the
remaining cases (using an enum in ListModel, and assigning
an enum to an integer property) should also work correctly.

Task-number: QTBUG-21679
Change-Id: Ibff13f0b94da94b18e2e3bae4aa6ba44e0fa944b
Reviewed-by: Chris Adams <christopher.adams@nokia.com>