Fix v8 handle management in QSGLoader

Fix v8 handle management in QSGLoader

Previously, setting the source to a remote url and specified some
initial property values while active was false, and then setting
active to true, would cause undefined behaviour.

This commit ensures that the handles are managed correctly, and that
an appropriate v8 context exists during object creation with initial
property values specified.

Task-number: QTBUG-21868
Change-Id: I101c6546537aa05aaeb420195aca670bc71f31e1
Reviewed-on: http://codereview.qt-project.org/6109
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>