QtQuick.Dialogs FileDialog: default mode is ExistingFile

Previously did not sync the mode with the state of the selectExisting
property on construction.
(cherry-picked from qtdeclarative/fa5db4201945565bd2c5ff42ab233f497666003f)

Task-number: QTBUG-35877
Change-Id: Ifb630fb51c463025ec2f6a1932714f354552c234
Reviewed-by: Liang Qi <liang.qi@digia.com>