Move \OC\Preview to PSR-4

lib/private/Preview/BMP.php

lib/private/Preview/Bitmap.php

lib/private/Preview/Font.php

lib/private/Preview/GIF.php

lib/private/Preview/Illustrator.php

lib/private/Preview/Image.php

lib/private/Preview/JPEG.php

lib/private/Preview/MP3.php

lib/private/Preview/MSOffice2003.php

lib/private/Preview/MSOffice2007.php

lib/private/Preview/MSOfficeDoc.php

lib/private/Preview/MarkDown.php

lib/private/Preview/Movie.php

lib/private/Preview/Office.php

lib/private/Preview/OpenDocument.php

lib/private/Preview/PDF.php

lib/private/Preview/PNG.php

lib/private/Preview/Photoshop.php

lib/private/Preview/Postscript.php

lib/private/Preview/Provider.php