Exports the inputed tree object to the given excel file.
        
        :param      tree     | <QTreeWidget>
                    filename | <str>
        
        :return     <bool>

def exportTree(self, tree, filename):
        """
        Exports the inputed tree object to the given excel file.
        
        :param      tree     | <QTreeWidget>
                    filename | <str>
        
        :return     <bool>
        """
        book = xlwt.Workbook()
        
        # determine the title for the sheet to export
        title = nativestring(tree.windowTitle())
        if not title:
            title = nativestring(tree.objectName())
        if not title:
            title = 'Sheet 1'
        
        sheet = book.add_sheet(title)
        
        # determine visible columns to export
        cols = [c for c in range(tree.columnCount()) \
                if not tree.isColumnHidden(c)]
        
        # export the column headers
        hitem = tree.headerItem()
        for c, col in enumerate(cols):
            sheet.write(0, c, nativestring(hitem.text(col)))
        
        self._currrow = 1
        for i in range(tree.topLevelItemCount()):
            self.exportTreeItem(sheet, cols, tree.topLevelItem(i))
        
        book.save(filename)
        return True