added function to retrieve a list of flattred things from flattr
added function to update database flattr status according to the flattr status retrieved from flattr
Retrieve things that have been flattred from the flattr api.

This commit contains the UI changes required to use the functions in 71973b130 and 3bb51e456.

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