OTHER: Rename AbracaMainWindow -> AbracaWindow.
OTHER: Handle missing volume channels better.

Volume channels may come and go during xmms2d lifetime.
When playback starts we perform an additional query to
make life easier for users of the volume API.
OTHER: Avoid deprecated APIs.
OTHER: Add -fdiagnostics-show-option compiler option.
OTHER: Expose more xmmsv_dict_iter_t functions.
OTHER: Modernize equalizer window.