Marks this widget as loading records.

def markLoadingStarted(self):
        """
        Marks this widget as loading records.
        """
        if self.isThreadEnabled():
            XLoaderWidget.start(self)
        
        if self.showTreePopup():
            tree = self.treePopupWidget()
            tree.setCursor(Qt.WaitCursor)
            tree.clear()
            tree.setUpdatesEnabled(False)
            tree.blockSignals(True)
            
            self._baseHints = (self.hint(), tree.hint())
            tree.setHint('Loading records...')
            self.setHint('Loading records...')
        else:
            self._baseHints = (self.hint(), '')
            self.setHint('Loading records...')
        
        self.setCursor(Qt.WaitCursor)
        self.blockSignals(True)
        self.setUpdatesEnabled(False)
        
        # prepare to load
        self.clear()
        use_dummy = not self.isRequired() or self.isCheckable()
        if use_dummy:
            self.addItem('')
        
        self.loadingStarted.emit()