wrong column order passed to sortChanged

[mabi]

When enabling external sort and selecting more than one column, the column order passed to sortChanged is not correct.
The cause is in compare function rowSorter.prioritySort

[mabi]

I already fixed this issue in my repository and I am working on a pull request.

[mabi]

PR is #5778
Travis CI fails, but since ten days ago all PRs fail the same tests, I think it is not related to my commit.