MessageDialog: press escape or titlebar close button to dismiss

QMessageBox is a bit special compared to other dialogs, but we don't
need it to be so special in QtQuick.Dialogs.  Also on OSX, cmd-period
will dismiss the dialog, regardless whether it is a QMessageBox or
the QML implementation.

Task-number: QTBUG-36227
Change-Id: I5dd43080600e9c3774990190bb401291efb8e742
Reviewed-by: Liang Qi <liang.qi@digia.com>