android notification instead of popup dialog when flattring queue
Up until now FlattrStatusFetcher was only used to mark things as flattrd, when AntennaPod didn't already know they had been flattrd by other means.

Now we also mark things as unflattrd if they are not among the things listed by the flattr API. Things where flattring failed (but antennapod thought it worked) are thus re-enabled upon a FlattrStatusFetcher run.

On the other hand, as the URI normalization is tricky and may not cover all the ugly ways in which podcasters or flattr specify URIs, this may yield false negatives
(things marked as unflattrd in AntennaPod that actually have been flattrd).

As discussed in danieloeh/AntennaPod#203 and danieloeh/AntennaPod#87