Biggie: PlaylistFile refactoring, code deduplication, asynchronous loading

FEATURES:
* Playlist files are now read asynchronously when possible; patch and great
deal of refactoring by Tatjana Gornak.

Original description:
I've started my changes with an implementation of a ASXPlaylist class,
due this work I've found that implementation of different playlist file
types has different logic, as result I end up with a rewriting
playlist's implementations.

As example of difference:
- Constructor M3UPlaylist::M3UPlaylist( const KUrl &url ) sets a url,
but does not load playlist, therefore loading of playlist are
postponed till data from playlist are actualy needed (lazy loading)
On the other hand constructor XSPFPlaylist::XSPFPlaylist( const KUrl
&url, bool autoAppend ) loads playlist.

The main idea of proposed changes is to create a unify code for
processing playlist files:
- lazy loading
- common functionality was moved to PlaylistFile.


FIXED-IN: 2.8
REVIEW: 107473
DIGEST: Optimization: Amarok now reads playlist files asynchronously
when possible, thanks to Tatjana Gornak.
CCMAIL: Tatjana Gornak