QtQuick.Dialogs.ColorDialog: set current color in the QColorDialog

It was a missing feature to set the current color programmatically,
either before or after showing the dialog.

Task-number: QTBUG-32621
Change-Id: I62a811f7c36eaab5356c5924bf3447deca55ada3
Reviewed-by: Liang Qi <liang.qi@digia.com>