Don't recalculate cachedir

Fixes #100
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
Remove self.player.nextUrl setting - this is not needed anymore
Merge pull request #149 from roignac/nextUrl_remove

Remove self.player.nextUrl setting - this is not needed anymore
player: Add new method get_current_media()
Move get_metadata from player to mpris
mpris: Don't use introspection mechanism for doubles and 64-bit integers

dbus-python might incorrectly introspect doubles and 64-bit integers, so
tell it explicitly the data types of those properties.
toolbar: Simplify update

- Use set_visible() to show/hide widgets
- Call set_custom_title() only once
- Hide back button if selection mode is enabled
Lower required version of GTK+
Use libgd widgets instead of gtk+ if the later is too old

Import Stack, StackTransitionType and StackSwitcher from libgd if gtk+ is older
than 3.9.
Use HeaderBar from libgd if not available in gtk+

Also, rename data/Headerbar.ui to data/headerbar.ui.in which configure will use
as the template for data/headerbar.ui file.

Create po/POTFILES.skip and add data/headerbar.ui to it.
toolbar: Don't set custom title on Toolbar init

This makes sure that the label created by the header is realized on init.
The setting of the state of the toolbar to 0 will do this anyway.