Missing UI files upon installation

Reported by Giorgos Keramidas:

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I recently started seeing tracebacks with Exaile trunk, throwing
errors like these:

0812 12:15 kobe:~/bzr/exaile$ exaile
INFO : Loading Exaile 0.3.3.0-dev...
INFO : Loading settings...
INFO : Loading plugins...
INFO : Loading collection...
INFO : Loading devices...
INFO : Loading interface...
INFO : Loading main window...
INFO : Connecting main window events...
INFO : Loading panels...
Traceback (most recent call last):
  File "/usr/local/lib/exaile/exaile.py", line 68, in <module>
    main()
  File "/usr/local/lib/exaile/exaile.py", line 65, in main
    exaile = main.Exaile()
  File "/usr/local/lib/exaile/xl/main.py", line 122, in __init__
    self.__init()
  File "/usr/local/lib/exaile/xl/main.py", line 256, in __init
    self.gui = xlgui.Main(self)
  File "/usr/local/lib/exaile/xlgui/__init__.py", line 102, in __init__
    exaile.collection, _show_collection_empty_message=True)
  File "/usr/local/lib/exaile/xlgui/panel/collection.py", line 210, in __init__
    panel.Panel.__init__(self, parent, name)
  File "/usr/local/lib/exaile/xlgui/panel/__init__.py", line 60, in __init__
    self.builder.add_from_file(ui_file)
TypeError: Gtk.Builder.add_from_file() argument 1 must be string, not None
Exception AttributeError: AttributeError("'NoneType' object has no
attribute 'remove_panel'",) in <bound method CollectionPanel.__del__
of <CollectionPanel object at 0x29faa00
(xlgui+panel+collection+CollectionPanel at 0x2bed740)>> ignored

Bisecting through the commits since the last version I had merged
locally into my copy, I noticed that this started with the following
commit:

0812 12:15 kobe:~/bzr/exaile$ bzr log -r 4030.1.14
------------------------------------------------------------
revno: 4030.1.14
committer: Mathias Brodala <email address hidden>
branch nick: exaile-0.3.x
timestamp: Thu 2012-08-09 01:34:19 +0200
message:
  * Move panel UI files into subdirectory

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The Makefile was not adapted to the additional directory.