Utils: Handle json_data == null case in show_error_object

...which can happen if there's no internet connection at all.
ITimeline: Don't show dialog if not connected
Add FavoritesTimeline
FavoritesTimeline: Set icon, label and tooltip
FavoritesTimeline: remove unneded cruft

... copied from HomeTimeline.
Corebird: Add Alt+6 accelerator
Corebird: code style
FavoritesTimeline: implement unfavoriting

...which is just like deleting here.