Allow enum values to be used as signal parameters

Allow enum values to be used as signal parameters

The fix does not work for enums declared outside of the class that
emits the signal, since in this case it is not possible to access
the metaobject of the class that declared the enum. However the
fix makes a special case for the Qt namespace to allow enums in this
namespace to be used.

Task-number: QTBUG-15983
Change-Id: I6f72255b07010311a20fe068bd97c7db7b294d9b
Reviewed-by: Aaron Kennedy