Bug#758023: important addition "GLib.UserDirectory.DIRECTORY_MUSIC"

[Dmitry Ostasevich]

Start fail has begun after I changed my locale from ru_RU.UTF-8 to
en_US.UTF-8, user directories were renamed either. This seems as the
reason of start fail as everything works fine under another user. Also
if I try to run gnome-music from root user I get:

Traceback (most recent call last):
  File "/usr/bin/gnome-music", line 38, in <module>
    from gnomemusic.application import Application
  File "/usr/lib/python3/dist-packages/gnomemusic/application.py", line
36, in <module>
    from gnomemusic.window import Window
  File "/usr/lib/python3/dist-packages/gnomemusic/window.py", line 38,
in <module>
    from gnomemusic.query import Query
  File "/usr/lib/python3/dist-packages/gnomemusic/query.py", line 32, in
<module>
    class Query():
  File "/usr/lib/python3/dist-packages/gnomemusic/query.py", line 35, in
Query
    GLib.get_user_special_dir(GLib.UserDirectory.DIRECTORY_MUSIC)
TypeError: Argument 0 does not allow None as a value