Support for Apple podcast frames

[Hasso Tepper]

https://github.com/hasso/taglib/commit/bdf06fb335759576521b816cceb6ddefa07c08ed

The change implements support for proprietary frames Apple iTunes uses to
tag podcast files. These frames are not documented anywhere and the info
is mostly gathered from various sources in the Internet.

Most of it is straightforward, the only question is about PCST frame. It
seems to be just a flag - it's always four bytes of zeros. I'm not sure
what treatment it would need in the taglib. And whether at all.

It's tested with my podcast collection - reorganizing into folders in
iTunes etc.


-- 
Hasso Tepper