grilo: Fix fallback icon for sources

When we fail to get the icon set for sources, make sure to use the
"box" icon, not the video one. Freebox' UPnP server points to a
non-existant file.
grilo: Resolve local metadata when adding new items

Resolve local metadata such as the thumbnail to use for sources
that don't provide thumbnails themselves. This adds thumbnails to
the Tracker added items.
grilo: More assertions for unsupported items