Wrong fallback order of mimetype icons in GIO

Beginning with the version of GIO present in Intrepid Beta, the icons for mimetypes have a wrong fallback order. This is caused by the following snippet in glib2.0-2.18.1/gio/gcontenttype.c, function g_content_type_get_icon:

  G_LOCK (gio_xdgmime);
  xdg_mimetype_icon = g_strdup (xdg_mime_get_icon (type));
  G_UNLOCK (gio_xdgmime);

  (...)

  if (xdg_mimetype_icon)
    icon_names[n++] = xdg_mimetype_icon;

  icon_names[n++] = mimetype_icon;
  icon_names[n++] = legacy_mimetype_icon;
  icon_names[n++] = generic_mimetype_icon;

xdg_mime_get_icon is supposed to get the custom name of the icon for the type specified by the user and defined in /usr/share/mime/icons. The problem is that xdg_mime_get_icon automatically falls back to the generic icon if there is no custom override defined by the user. As a result all mimetypes that have a generic icon assigned completely ignore specific mime icons from the theme (e.g. application-x-rar) and use the generic icon instead (e.g. package-x-generic). This is described in more detail in bug #279911 and bug #278113. Please note that this problem is completely different from bug #217997, because we're speaking about generic icons overriding specific icons from the same theme, not generic icons from current theme overriding hicolor icons.

The fix is to remove this fallback mechanism from xdg_mime_get_icon, and modify the above snippet to also use xdg_mime_get_generic_icon and use the correct fallback order (1. XDG override, 2. specific mimetype icon, 3. legacy specific gnome-mime- icon, 4. XDG generic icon, 5. generic mimetype icon (e.g. application-x-generic). The attached patch does this. Note that xdg_mime_get_icon is only used in gcontenttype.c and only once, so the change won't introduce any regressions.