X.L.LayoutHints stop windows from overlapping

[ankaan]

Description

This is an improvement on the pull request "Fix render order of
LayoutHints and MultiColumns" (#186) and addresses the actual underlying
problem.

It turned out that windows can sometimes overlap also. This happens when
a window is exactly in the center along an axis. There is a special
case in the code for this, but it was not handled properly.

I removed the code that places the focused window on top since it is no
longer required, but I still preserve the window order of the underlying
layout. This interferes even less with the underlying layout.

I also removed some code paths that were no longer necessary due to this
change and generalized some types so that I could debug the code more
easily.

Checklist

• I've read CONTRIBUTING.md

• I tested my changes with xmonad-testing

• I updated the CHANGES.md file

[mention-bot]

@ankaan, thanks for your PR! By analyzing the history of the files in this pull request, we identified @pjones, @aavogt and @liskin to be potential reviewers.

[ankaan]

I realized that the special case that I initially removed actually made things more visually pleasing, so I added it back but made it work properly.

I also removed a comment that was wrong (and contradicted what is written right before it.)

[slotThe]

LGTM, but I don't really use layout hints, so I can't adequately review it in detail.

@liskin you were flagged by that bot in the first run; do you still use this functionality and can review the change?

@ankaan can you perhaps rebase on top of current master?

[liskin]

I do use LayoutHints but I don't use LayoutHintsToCenter so I'm probably unaffected, and it's been almost 10 years since I last touched this module, so I'm as qualified to review it as anyone else. Might be nice to add unit test for the problem being fixed, or something like that which will help potential reviewers to understand it.