Better caching for document textures and information

Move thumbnail creation into ShellTextureCache.  It's now asynchronous,
and we cache the result.

Create a DocManager class which keeps around the DocInfo objects between
invocations.  This is also where we ensure we remove thumbnails for
recent items not known anymore.

src/shell-global.h

@@ -36,8 +36,6 @@ GType            shell_global_get_type            (void) G_GNUC_CONST;
 gboolean shell_clutter_texture_set_from_pixbuf (ClutterTexture *texture,
                                                 GdkPixbuf      *pixbuf);
 
-GdkPixbuf *shell_get_thumbnail(const gchar *uri, const gchar *mime_type);
-
 guint16 shell_get_event_key_symbol(ClutterEvent *event);
 
 guint16 shell_get_button_event_click_count(ClutterEvent *event);