Revert "qquickrangemodel: don't emit signals before component is completed"

This reverts commit 774f2aa1

 that
broke sliders (auto test for RangeModel included to prevent that
from happening again) in the Qt Quick Enterprise Controls gallery.
Some sliders with a hard-coded range [0..360] and value 240 ended
up having a value of 0 at Component.onCompleted.

Change-Id: Ib3024c51c0552ac06528ba47b08e1a33a5f48324
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>