Get the real index of the selected item.

def get_stack_index(self, stack_index, plugin_index):
        """Get the real index of the selected item."""
        other_plugins_count = sum([other_tabs[0].count() \
                                   for other_tabs in \
                                   self.plugins_tabs[:plugin_index]])
        real_index = stack_index - other_plugins_count

        return real_index