fix ctkFileDialog::setAcceptButtonEnable with setAcceptMode

If QFileDialog::setAcceptMode() is called, all the buttons are deleted.
Which breaks the mechanism in place by AcceptButtonEnable.
The fix consists of observing the dialog box for the addition of the
accept button as a child.
Merge branch 'ctkfiledialog-setfilemode'

* ctkfiledialog-setfilemode:
  fix ctkFileDialog::setAcceptButtonEnable with setAcceptMode