Collapses all triggers and makes all blocks with fold level > 0
        invisible.

def collapse_all(self):
        """
        Collapses all triggers and makes all blocks with fold level > 0
        invisible.
        """
        self._clear_block_deco()
        block = self.editor.document().firstBlock()
        last = self.editor.document().lastBlock()
        while block.isValid():
            lvl = TextBlockHelper.get_fold_lvl(block)
            trigger = TextBlockHelper.is_fold_trigger(block)
            if trigger:
                if lvl == 0:
                    self._show_previous_blank_lines(block)
                TextBlockHelper.set_collapsed(block, True)
            block.setVisible(lvl == 0)
            if block == last and block.text().strip() == '':
                block.setVisible(True)
                self._show_previous_blank_lines(block)
            block = block.next()
        self._refresh_editor_and_scrollbars()
        tc = self.editor.textCursor()
        tc.movePosition(tc.Start)
        self.editor.setTextCursor(tc)
        self.collapse_all_triggered.emit()