qmljs: coalesce resets of code model triggered by cpp files update

Use a timer to avoid resetting the model too often.

Change-Id: Iadf3ff3f03cfc91df44168243972b1e5a0ee5077
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>