Ability to add all my songs to playlist - Issue #199

I've added this feature for myself, submitting a pull request in case you find this useful!

This creates an (ALL) option at the top of the FileSystem library type.  Long pressing it will allow you to add all songs to the current/specified playlist.  It will only appear at the root directory level and not in any subdirectories.  It will not appear in any other library types.

I've also added a `getId()` method to the `Item` class, so that the (ALL) behavior can be implemented in subdirectories or other library types (album, artist, etc.)  

Preview:

![Screenshot from 2013-03-24 19:44:19](https://f.cloud.github.com/assets/746276/295848/5126699a-94bb-11e2-9042-1a2e78fd1f90.png)