Add HiddenCheckbox widget
Add HiddenCheckbox widget

Also fix error in HiddenInputs widget when not providing any data.

DirkHoffmann/indico,ThiefMaster/indico,pferreir/indico,OmeGak/indico,OmeGak/indico,pferreir/indico,ThiefMaster/indico,pferreir/indico,mvidalgarcia/indico,indico/indico,OmeGak/indico,DirkHoffmann/indico,pferreir/indico,DirkHoffmann/indico,indico/indico,mic4ael/indico,OmeGak/indico,indico/indico,mvidalgarcia/indico,ThiefMaster/indico,mvidalgarcia/indico,mic4ael/indico,ThiefMaster/indico,indico/indico,DirkHoffmann/indico,mic4ael/indico,mvidalgarcia/indico,mic4ael/indico