Allow invokable functions of value-type classes to be called

Allow invokable functions of value-type classes to be called

Previously, invokable functions of value-type classes were returned as
properties.  This commit fixes that bug by allowing such functions to
be invoked normally.  It also improves copy-value type handling.

This commit also ensures that QMatrix4x4 value types are constructed
with qreal values as this is the storage type used internally.

Change-Id: Iab0fe4c522ed53d60154e8a8d46dda925fb9f4de
Reviewed-by: Martin Jones <martin.jones@nokia.com>