glib: don't crash when clicking on artist/songs tab

Use empty string and not None to fix following exception

Traceback (most recent call last):
  File "/usr/lib/python3.3/site-packages/gnomemusic/grilo.py", line 72,
in _callback
    callback(source, param, item)
  File "/usr/lib/python3.3/site-packages/gnomemusic/widgets.py", line
502, in get_songs
    title = AlbumArtCache.get_media_title(track)
  File "/usr/lib/python3.3/site-packages/gnomemusic/albumArtCache.py",
line 209, in get_media_title
    title = GLib.uri_unescape_string(basename, None)
  File "/usr/lib/python3.3/site-packages/gi/types.py", line 113, in
function
    return info.invoke(*args, **kwargs)
TypeError: Argument 1 does not allow None as a value
Revert "Don't recalculate cachedir"

This reverts commit 017fc7e08dae8f6cededf514de64e70eb7dbe497.
Add GMenu Actions
We do not need to register the Gio.Resource
Merge pull request #146 from lubosz/master

glib: don't crash when clicking on artist/songs tab
Fix crash when a track isn't loaded on ArtistAlbumWidget.get_songs

If you switch very fast from the 'Albuns' view to the 'Artists'
view, it crashes. Apparently it happens when a track is not loaded
yet, so methods such as get_url() return NoneType which cannot be
converted to 'str' implicitly. It needs to be converted to 'str'
explicitly to be used on logging.debug.
Let's leave 'print' for now - until we implement proper logging
Translate 'All Artists' label

http://bugzilla.gnome.org/show_bug.cgi?id=705505
Merge pull request #147 from felipeborges/crash-artists-view

Fix crash when a track isn't loaded on ArtistAlbumWidget.get_songs
Don't recreate cacheDir in each instance of AlbumArtCache