New collection scanner

[Ralf Engels]

Hi all,
since this morning (very early) the new collection scanner is integrated.

It should allow us to implement exiting new features and it also has some benefits over the old scanner.

However the change is quite big and we even might want to revert the fix if serious problems come up.

For now a short overview about the new features and already known problems.

- The new scanner should be smarter in figuring out if an album is a compilation or not.
 If "compilation" is set by the user this information should also be saved and used in the next scanning run.
- Rating, playcount and score should be saved and restored.
- The playlist and collection view should be always up-to-date. There might be an empty album if all songs are removed from one. 
 There should however never be a case where right-clicking on a track does not bring up a context menu.
- The scanner should be a lot smarter in detecting which cover belongs to which album.

known issues:
- Probably the most severe issue right now: The scanning seems to hang sometimes.
 Sometimes meaning once every four full scans on my machine.
 If you have such a problem: please send me the last few debug output
 lines from the scanner.
- The progress bar is not progressing to 100%
 Actually I couldn't really figure out yet why this happens but
 I will eventually.
- The scanner will mark an album with a single track as compilation more often than the old one did.
 I am still not sure if this is good or bad. For now I think it's a little
 annoying that a lot of my singles appear under "various artists". 
 So, if you have any opinions please drop me a message.
- If you see two albums with the same name and artist that appear in the collection.
 That is probably an album with two different album artists and not
 directly connected to the scanner.

performance:
- I have done perfomance tests with the scanner processor but not with the whole scanner as such.
 The scanner is reading more information than before but that shouldn't slow it down by much.
 However lfranchi has reported an increased scanning time.
 If you also have this impression please try to give me some numbers.
 The scanning time of 20000 mp3 files on a local disk should be below three minutes.
 

Any new bugs, complaints or improvement request can be directed to me directly, IRC, bug tracker or as a patch on the review board.
As always.

Cheers,
Ralf