impossible to load cd (device) panel

r4047

Just an oversight after moving and renaming some ui files.

Trying to load the cd plugin:

Traceback (most recent call last):
  File "/home/***/exaile/xlgui/guiutil.py", line 37, in _idle_callback
    value = func(*args, **kwargs)
  File "/home/***/exaile/xlgui/__init__.py", line 356, in add_device_panel
    device, device.get_name())
  File "/home/***/exaile/plugins/cd/_cdguipanel.py", line 75, in __init__
    device.FlatPlaylistDevicePanel.__init__(self, *args)
  File "/home/***/exaile/xlgui/panel/device.py", line 161, in __init__
    panel.Panel.__init__(self, name)
  File "/home/***/exaile/xlgui/panel/__init__.py", line 61, in __init__
    self.builder.add_from_file(ui_file)
TypeError: Gtk.Builder.add_from_file() argument 1 must be string, not None