Fix black screen when stop playing a playlist

This was because we alway set the currentPlaylist when the play queue
was refreshed.

Fixes #1126
Fix crash when opening context menu on a music item
Fix so that MusicOSD can be visible on NowPlaying
Added google mock
Removed dead code
Add support for ListItem.CompositeImage(args)

This accepts the composite image call arguments.

Fixes #1121
Add Plex.PlayAll / Plex.ShuffleAll to the commands list.

Related to #1107
Always refresh playQueue fanout.

Avoids the playQueue list being stuck.
Kill API calls to /library/sections on plex.tv
Remove our own Art methods in GUIListItem

We don’t need this anymore with the composite image stuff.
Send PLEX_PLAYQUEUE_UPDATED message more often

Fixes issues with the fanout not being updated.
Make sure to hide the busy dialog if the navigation fails

This makes the navigation go away when trying to navigate past the last
episode in the list.