Align QVariant::UserType and QMetaType::User

Align QVariant::UserType and QMetaType::User

There is no point in keeping separate values which should mean the
same.

QVariant::UserType was used also to construct a valid, null QVariant,
containing an instance of unknown custom type. The concept was strange
and useless as there was no operation that could be done on such
QVariant. Therefore it was dropped.

Please note that the patch slightly changes behavior of different
functions accepting a type id as parameter. Before QVariant::UserType
was an invalid type from QMetaType perspective (id 127 was not assigned
to any built-in type), but QMetaType::User points to the first registered
custom type.

Change-Id: I5c7d541a9affdcdacf53a4eda2272bdafaa87b71
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Andrew Stanley-Jones <andrew.stanley-jones@nokia.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>