MixDeviceWidget and subclasses: Fix declarations of signals

Eliminates the compile message

  gui/mixdevicewidget.h:77: Warning: Signals cannot be declared virtual

and the runtime message

  kmix/default QMetaObjectPrivate::indexOfSignalRelative: QMetaObject::indexOfSignal:
  signal guiVisibilityChange(MixDeviceWidget*,bool) from MixDeviceWidget redefined
  in MDWSlider