QSGLoader shouldn't load component when active is false

QSGLoader shouldn't load component when active is false

Previously, QSGLoader still loaded the component specified, but didn't
instantiate the item. This commit ensures that no component is loaded
from the source, and that the onLoaded signal is emitted only when
loading occurs (when the loader is active).

Task-number: QTBUG-21710
Change-Id: I2d83603ef84d6942fb84141e9e146d2cf9654fc4
Reviewed-on: http://codereview.qt-project.org/5915
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>