New panel: library

Aiming to display library by Genre/artist/album
Load and display library (WIP)
[WIP] manage library filters

Also change "browse" icon
Lib filtering artist/album/genre

Allow browsing by genre/artist/album
[WIP] library: sort lists, play/enqueue buttons
Fix overlapping divs

With z-index rather than margin, to avoid horizontal scrollbar appears
Library: play, enqueue, play all, enqueue all

Had to hack around when sending MPD commands, due to MPD server apparently not understanding many commands closely sent