By Matt Siegel SYDNEY (Reuters) - Militants connected with radical group Islamic State were planning to behead a member of the public in Australia, Prime Minister Tony Abbott said on Thursday, after hundreds of police raided homes in a sweeping counter-terrorism operation. Abbott said there was a "serious risk from a terrorist attack" days after Australia raised its national terror threat level to "high" for the first time, citing the likelihood of attacks by Australians radicalised in Iraq or Syria. ...