We don't ship an icon for libre.fm, so fall back to a default in order
to avoid displaying a not found icon in the sidebar. We attempted to do
this by constructing a GThemedIcon with both the desired name
("Libre.fm-symbolic") and the fallback.

Unfortunately, with this method (once the 'len' parameter to
g_themed_icon_new_for_names () is fixed - another bug), we always get
the fallback icon.

To fix this, we can look the icon up in the Gtk icon theme and only use
it if it is found, falling back otherwise.