ENH: DataFrame sort columns by rows: sort_values(axis=1)
- [X] closes #10806
- [X] tests added / passed
- [X] passes `git diff upstream/master | flake8 --diff`
- [X] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13622?src=pr) is **84.38%**

> Merging [#13622](https://codecov.io/gh/pydata/pandas/pull/13622?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.04%**

``` diff
@@             master     #13622   diff @@
==========================================
  Files           138        142     +4   
  Lines         51100      51222   +122   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43097      43223   +126   
+ Misses         8003       7999     -4   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [65849d3...ea2d89e](https://codecov.io/gh/pydata/pandas/compare/65849d3c9feea395d6e6a124f7a3b11ecdb943cb...ea2d89eb82a915d522d783c35e1759b826821713?src=pr)
 Looks good. @jreback OK for you?
 ok minor comments, ps rebase and ping on green.
 @IamJeffG fixed the remaining comments when merging (we already asked you to update the whatsnew sentence enough :-)) 
Thanks a lot for the enhancement!
 Thanks for taking care of it @jorisvandenbossche !
 (and now your geopandas PR .. :-) I try to take a look this evening!)
