Fixes hide_playlist_targets not working after server restart

[datasone]

Currently, hide_playlists_targets works by skipping songs with in_playlist value set to true in Directory::Walk. But in_playlist is not stored and only updated in UpdateWalk::PurgeDanglingFromPlaylists, which will only be executed while updating DB.

This causes the problem that playlist target songs are correctly hidden after database update, but will remain visible after mpd server restarted. This pr solves the problem by storing in_playlist value of songs into the SimpleDatabase file.

[MaxKellermann]

838b19c
Cherry-picked and amended with two changes:

• the commit message was bad; you wrote text in this PR but it was missing in the commit message; please do not wrote PR texts, because these explanations are lost after merging; all that counts is the commit message
• I changed your code to only emit the InPlaylist line if the value is yes; writing it always just adds unnecessary clutter if the feature is not used