Use isaudio instead of hasitems

to determine whether a plugin item is can be played.

This fixes issue 54:
https://code.google.com/p/android-squeezer/issues/detail?id=54
and possible issue 86, and others.
Change toString method of model classes

to make debugging easier
ItemAdapter now supports isEnabled

isEnabled is delegated to ItemView. The default implementation returns
true (like BaseAdapter does).

Currently used to disable plugin items without sub-items.

Also fix some comment problems in ItemAdapter, and remove the unnecessary
"public" from some ItemView method declarations.
Disable items with no action in onItemSelected

- Playlist item, if the selected action is NONE
- If the musicfolder item is a "track" and the selected playlist item
  action is NONE or the musicfolder item is not a "folder".