macosx: Spacebar toggles audio playback

Fixing an issue where the spacebar hotkey would not pause or resume playback for audio-only media. Works fine for video but not audio files. The event was being consumed by the main VLCLibraryWindow but was not being forwarded to the playback controller because the hotkey handling logic was only connected to the video view (VLCVoutView).
My approach was to centralize the VLCHotkeysController in the VLCMain singleton so as to be able to use it in VLCLibraryWindow.m to handle toggling audio playback whenever space bar is pressed