Unwrap QJSValue from QVariant in QV8Engine::fromVariant

Unwrap QJSValue from QVariant in QV8Engine::fromVariant

When QML tries to unwrap real value from a QVariant and
the value is a QJSValue instance, then no conversion is needed,
QJSValue already contains a v8 handle.

This patch, for example, solves a problem of emitting QJSValue
instance in a signal that has QVariant as an argument. The QJSValue
can be unwrapped and used as a normal JS value in a connected slot.
This feature may be used also in a plugin model that stores QJSValues
internally. Then the model in data() function can return a QJSValue
which would be understood by QML.

Change-Id: I1d5ede40ce2637123b09839fd848b27ad3af3dda
Reviewed-on: http://codereview.qt-project.org/4451
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>