Sets whether or not this combobox stores checkable items.
        
        :param      state | <bool>

def setCheckable( self, state ):
        """
        Sets whether or not this combobox stores checkable items.
        
        :param      state | <bool>
        """
        self._checkable = state
        
        # need to be editable to be checkable
        edit = self.lineEdit()
        if state:
            self.setEditable(True)
            edit.setReadOnly(True)
            
            # create connections
            model = self.model()
            model.rowsInserted.connect(self.adjustCheckState)
            model.dataChanged.connect(self.updateCheckedText)
        
        elif edit:
            edit.setReadOnly(False)
        
        self.updateCheckState()
        self.updateCheckedText()