* added the opaqueness-slider to the extended panel (refs #318)

* added the opaqueness-slider to the extended panel (refs #318)
    - on-the-fly changing is not yet possible, since I don't know the correct pointer (the current one gives "dereferencing pointer to incomplete type"); see FIXME.
* VLCExtended is the delegate for o_extended_window now, to save the prefs automatically, once VLC is going to be terminated
* "awakeFromNib" was partially moved to "showPanel" to retrieve the slider values before showing the window, in case they were saved in the prefs-window