BUG: Stacking with multiple mixed int/str levels

Add test case for mixed type stacking

Used wrong var name in the assert

Method to swap levels assuming ints are level numbers

Fix _stack_multi_columns to deal with mixed strs/ints

Extra testcases

Add fix to the release notes

Convert to label before swaplevel if possible

Revert "Method to swap levels assuming ints are level numbers"

This reverts commit 61f96fd3cb23cda9f9c7a6837b145ebd247a55cc.

More test cases

Use _convert_level_number() to sort columns
Merge pull request #8809 from onesandzeroes/stackfix

BUG: Passing multiple levels to stack when having mixed integer/string level names (#8584)