qml: provide modal dialogs instantiable from QML

  This dialog allows to ask simple Yes/No questions, such as action
  confirmation, with a simple callback mechanism.

  The modal dialog need to be attached to the root widget to be displayed
  properly.

  Using vlc_dialog_wait_question for this would block the UI.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>