Add initial support for multiple libraris

- new table library_track stores relation of tracks and libraries
- albums/titles/genres etc. queries can filter by library

A plugin "LibraryDemo" would create a virtual library for tracks longer
than 10 minutes. It's rather simplistic, but shows two ways to get the
library values in to the db (one for the standalone scanner, one for the
LMS internal scanner). Two menu items for albums and artists would be
added to My Music.
Merge branch 'public/7.9' into public/feature/multi_library

Change-Id: Iabfa9d012ae797df51382973e2b3cf43f2c23fbf
Need to initialize LibraryTrack

Change-Id: Ib32db3263f61dedb5b212a61d4107889d6c5ad96
Merge branch 'public/7.9' into public/feature/multi_library
Add playlist_id support to playlist queries

Change-Id: I55bc852a94e50a50c3d05e2fa20a3cd13838f8e8
Add support for global virtual library handling. BrowseModes would read
a pref library_id whenever called to limit results to subsets of the
complete library.

The library_id can be a per-player pref or globally set. There's no code
yet to set any of this values.

Please note that you'll need to DELETE the library.db file and run a
full rescan for this change to work. There has been another schema
change I don't want to create a new schema definition file for. You're
living on the edge ;-).
Add "Library View" menu item to switch between virtual libraries.

TODO: need to take care of various caches when the library_id pref is
changed.
Various caches need to be aware of virtual libraries. 

Rewrite some code to re-use more common code. Implementing the same
thing in many different places is stupid. Calculating the totals might
be slightly slower than before, but it's more accurate with less added
complexity. And their only calculated once, then cached.
Add an icon to the browse mode selector menu item.
RandomPlay shall respect libraries, too.
Update LibraryDemo with some more interesting virtual libraries.
Merge branch 'public/feature/multi_library' into public/7.9
Enalbe the LibraryDemo plugin for now. We want this to be tested.